Gram negative bacteria in microbiology examples, list, and structure

The Gram staining procedure in microbiology is a differential staining technique used to differentiate bacterial cells into gram-positive or gram-negative bacteria based on the differences in their cell wall components. At the end of the gram stain, gram negative bacteria stain pink/ red while gram positive bacteria stain purple or blue. Gram-negative bacteria are characterized by their cell envelopes. The cell envelope of the gram negative bacteria cell is made up of a thin peptidoglycan cell wall incorporated between an inner cytoplasmic cell membrane and an outer membrane.

These organisms can be found in virtually all environments on Earth that support life. Common gram negative bacteria representative species include the model organism, Escherichia coli, and many pathogenic bacteria, such as Chlamydia trachomatis, Pseudomonas aeruginosa, and Yersinia pestis. These gram negative bacteria examples are of medical significance because their outer membrane protects them from lysozyme, detergents and many antibiotics (including penicillin). Lysozyme is an antimicrobial enzyme produced by animals that form part of the immune system.

Mechanism of the Gram stain test for bacteria classification

According to the gram stain test, bacteria are traditionally divided into two groups:- gram-positive and gram-negative. The gram staining response of bacterial cells is an empirical criterion making gram positive bacteria and gram negative bacteria the two types of bacteria. This differential staining method is based on the distinct differences in the structure and composition of the bacterial cell wall. This difference is due to the presence or absence of an outer lipid membrane. Therefore, the two main features that differ the structure of gram positive bacteria from gram-negative bacteria include the thickness of the peptidoglycan layer and the presence or absence of the outer lipid membrane.

The gram staining procedure involves 4 consecutive steps which include:

1. The application of a primary stain (crystal violet)
2. Addition of a mordant (iodine)
3. Decolorizing with alcohol
4. Counterstaining

During this process, it is the cell wall structure that affects the ability of the bacterial cell to retain or lose the crystal violet stain used in the Gram staining procedure. Gram-negative bacteria are classified based on the color they turn after this gram staining process. After the gram staining process, the gram negative bacteria stain red or pink. This is because they take up the stain of the counterstain after being decolorized. The other bacterial cells that stain purple or blue are called gram-positive bacteria. Gram-positive and gram negative bacteria stain differently in a gram stain test because of the difference in their cell wall composition. The color of the stained bacteria after the gram stain test is what differentiates gram positive bacteria from gram negative bacteria.

During the gram stain procedure, gram positive bacterial cells take up the crystal violet stain used, thus, appearing purple/ blue under the microscope. This is attributed to the cell wall structure of these bacteria. The cell walls of gram positive bacteria contain thick peptidoglycan layers. The peptidoglycan layer retains the primary stain used even after the decolorization stage of the gram staining steps. This is why they appear purple or blue after the gram stain procedure.

The gram-negative bacteria, on the other hand, are unable to retain the crystal violet stain after the decolorization stage because the alcohol used in this stage degrades the outer membrane of gram-negative bacteria. This makes the cell wall of gram negative bacteria more porous and unable to retain the crystal violet stain. The peptidoglycan layer of the gram-negative cell is much thinner and sandwiched between an inner cell membrane and an outer membrane. This is why they take up the counterstain (safranin or fuchsine) color. Hence, after the gram stain, gram negative bacteria color appears to be red or pink under the microscope. In conclusion, what brings about the difference between gram positive and gram negative bacteria staining color is the proportion of peptidoglycan in their cell wall.

Characteristics of gram negative bacteria

1. Gram negative bacteria under the microscope appear red or pink after the gram staining test.
2. They possess an inner cell membrane (cytoplasmic membrane).
3. Presence of a thin peptidoglycan layer. This is one of the gram negative characteristics that bring about the difference between gram positive and gram negative bacteria as the peptidoglycan layer is much thicker in gram-positive cells.
4. The gram negative bacteria cell has an outer membrane that contains lipopolysaccharides (LPS). This outer membrane structure is another feature that marks the difference between gram positive and gram negative bacteria.
5. The LPS in the outer membrane of gram negative bacteria consists of lipid A, core polysaccharide, and O antigen) in its outer leaflet and phospholipids in the inner leaflet.
6. One of the distinct characteristics of gram negative bacteria is the periplasm that fills the space existing between the outer membrane and the cytoplasmic membrane. This periplasm is a concentrated gel-like substance.
7. In the gram negative bacteria cell, the S-layer is directly attached to the outer membrane instead of the peptidoglycan.
8. One of the gram negative bacteria characteristics is the presence of porins in the outer membrane of gram-negative bacteria. These porins act like pores for specific molecules.
9. The gram negative bacteria with flagella have four supporting rings rather than two.
10. Teichoic acids or lipoteichoic acids are absent in gram negative cells. This molecules are only present in gram positive bacteria.
11. The Lipoproteins in gram-negative bacteria are attached to the polysaccharide backbone.
12. Some of these bacteria contain Braun’s lipoprotein. This molecule functions as a link between the outer membrane and the peptidoglycan chain by a covalent bond.
13. Gram negative bacteria shape could be spherical, rod-like or comma shaped.
14. Majority of gram-negative bacteria with few exceptions, do not form spores.

Gram negative bacteria structure

• Capsule
• Cell envelope
• Cell wall
• Cytoplasmic membrane
• Outer membrane
• Cytoplasm
• Plasmid
• Nucleoid
• Flagella
• Ribosomes
• Pilli

Structurally, the major difference between gram-positive and gram negative bacteria is the thickness of the peptidoglycan layer and the presence or absence of the outer lipid membrane. The cell walls of Gram-positive bacteria are made of thick layers of peptidoglycan compared to gram-negative bacterial cells.

The peptidoglycan layer of the gram-negative bacteria cell wall is 2-3 nm thick and covered with an outer lipid bilayer membrane whereas the gram-positive cell wall is a 20-80 nm thick polymer and has no outer membrane.

Nevertheless, despite gram-positive bacteria having a thicker peptidoglycan layer, gram-negative bacteria are more resistant to certain cell walls targeting antibiotics than gram-positive bacteria. This is due to the presence of an outer membrane in the gram-negative bacterial cell.

Capsule

A capsule is a third protective covering in some bacteria. This capsule is composed of polysaccharides and gram-negative bacteria are enclosed in it. The main function of the capsule in these bacteria is to prevent the white blood cells (that fight infection) from ingesting them. It also helps to keep the bacterium from drying out. The capsule as a gram negative bacteria structure is a major virulence factor in pathogenic gram-negative bacteria such as Escherichia coli.

Gram-negative bacteria have an outer membrane under this capsule. It is this outer membrane that protects them from some antibiotics such as penicillin. Once this membrane is disrupted it releases endotoxins (toxic substances) that contribute to the severity of the symptoms associated with gram-negative bacteria.

Cell envelope

The cell envelopes of gram-negative bacteria are composed of a thin peptidoglycan cell wall that is incorporated between an inner cytoplasmic cell membrane and an outer membrane.

Gram negative bacteria cell wall

The cell wall of gram negative bacteria is a crucial structure of these cells. There is a difference in cell wall structure between a gram positive and gram negative bacteria. The cell wall of gram negative bacteria is thinner than gram-positive cell walls. Moreso, the Gram-negative bacteria peptidoglycan layer is 2-3 nm thick and makes up 5-10% of the cell wall.

In gram negative bacteria cells, the cell wall has a second cell membrane (outer membrane) that is superficial to the peptidoglycan layer and adjacent to the inner cytoplasmic membrane. The outer membrane is located outside the gram negative bacteria peptidoglycan layer and is made up of a lipid bilayer. After the gram staining procedure, it is this cell wall structure that makes the appearance of gram negative bacteria under the microscope pink or red.

There is Braun’s lipoprotein (BLP, Lpp, murein lipoprotein, or major outer membrane lipoprotein) found in the cell walls of some gram-negative bacteria. These BLPs are very small and are one of the most abundant membrane proteins. They are covalently joined to the peptidoglycan layer and with their hydrophobic end, they are incorporated into the outer membrane.

Outer membrane of gram negative bacteria

One distinct gram-negative bacteria structure is the outer membrane. The outer membrane of gram negative bacteria protects them from several antibiotics (including penicillin), detergents, and lysozyme (antimicrobial enzyme). Lysozyme is an enzyme produced by animals that form part of the immune system.

The inner face of the outer membrane consists of phospholipids. Whereas, the outer leaflet of the outer membrane consists of a complex lipopolysaccharide (LPS) whose lipid A component can cause a toxic reaction when bacteria are lysed by immune cells. This toxic reaction can result in respiratory failure, low blood pressure, reduced oxygen delivery, and lactic acidosis.

Furthermore, the outer membrane of gram negative bacteria is made up of a protein, lipid bilayer, and an LPS layer. It is located outside the peptidoglycan layer and its lipid bilayer is intercalated with proteins, making the outer membrane superficial to the plasma membrane. The proteins found in the outer membrane include the porins and outer membrane proteins.

Lipopolysaccharides (LPS)

Lipopolysaccharides are the large molecules in the outer membrane of the gram negative bacteria cell wall. These LPS are anchored into the outer membrane and protrude into the environment from the cell. The chemical structure of LPS plays a role in many of the antigenic properties of gram-negative bacteria strains. These LPS in gram-negative bacteria are endotoxins and are attached to the outer membrane by a hydrophobic bond.

There are three different components that make up the LPS:

1. O-antigen or O-polysaccharide (the outermost part of the structure)
2. The core polysaccharide
3. Lipid A that anchors the LPS into the outer membrane

The function of LPS in gram-negative bacteria cell include the following:

1. It contributes to the net negative charge for the gram negative bacteria cell.
2. LPS helps to stabilize the outer membrane of gram negative bacteria.
3. The LPS gives the gram negative cell protection from certain chemical substances by blocking access to other parts of the gram negative bacteria cell wall.
4. It functions in the host response to pathogenic gram-negative bacteria.

Gram negative bacteria membrane

The gram negative bacteria membrane is also known as the cytoplasmic membrane of these bacteria. This membrane is a layer of phospholipids and proteins that encloses the interior of the bacterium. It functions in regulating the flow of materials in and out of the gram-negative cell. This particular gram-negative bacteria structure is a common trait with all other living cells. The cytoplasmic membrane is a barrier that allows cells to selectively interact with their environment.

Cytoplasm

The cytoplasm of gram negative bacteria is a gel-like matrix that contains other cell structures. The ribosomes, a chromosome, and plasmids are some of the main structures embedded in the cytoplasm. This cytoplasm is made up of water, nutrients, enzymes, gases, and wastes. Moreso, this matrix of the bacterial cell is where the metabolism, growth, and replication of the cell takes place.

The cell envelope of gram-negative bacteria encloses the cytoplasm and its components. It is important to note that bacterial cells don’t have a membrane-enclosed nucleus and so their chromosome (a single, continuous strand of DNA) is located in the nucleoid of the cytoplasm. The other cellular components of the cell are then scattered throughout the cytoplasm.

Plasmid

The plasmids are similar to the chromosomes as they are made of a circular piece of DNA. Plasmids are small extrachromosomal genetic structures. They can be seen in many strains of bacterial cells. However, these genetic structures are not involved in reproduction as chromosomes. The chromosomes are the only structure that has the genetic instructions responsible for starting and carrying out binary fission or cell division.

The plasmid is independent of the chromosome and replicate. Though plasmids are not essential for survival, they tend to give bacterial cells a selective advantage. Plasmids are passed on to other bacteria during binary fission, where copies of the plasmids in the cytoplasm are passed on to daughter cells or during conjugation, a process whereby bacteria exchange genetic information. During conjugation, a tubelike structure called a pilus passes copies of the plasmids to other bacterial cells.

Plasmids in both gram positive bacteria and gram negative bacteria are instruments in the transmission of special properties, such as antibiotic drug resistance, resistance to heavy metals, and virulence factors essential for the infection host organisms.

Nucleoid

The nucleoid of gram negative bacteria is the region of the cytoplasm where the chromosomal DNA is located. This is not a membrane-bound nucleus but an area of the cytoplasm where the strands of DNA are localized. Most bacteria have a single, circular chromosome for replication, while few species have two or more.

Flagella

Flagella are whiplike structures in bacterial cells that provide a means of movement. In order for the bacterium to move, the flagella structure beats in a propeller-like motion. The majority of gram positive bacteria and gram negative bacteria make use of this structure to move away from toxic chemicals, or move toward nutrients.

Ribosomes

The ribosomes in gram negative bacterial cells translate the genetic code from the molecular language of nucleic acid to that of amino acids. Amino acid is the building block of the proteins in the cell. These proteins perform all the functions of cells and living organisms. The ribosomes of bacteria are smaller than the ribosomes of eukaryotic cells and are slightly different in composition and molecular structure. In bacterial cells, the ribosomes are not bound to other organelles as in eukaryotic cells, instead, they are free-standing and distributed around the cytoplasm of bacterial cells.

Pili

Pili are the hair-like structures that project from the outside cell surface. The majority of bacterial cells have pili ( singular, pilus). This outgrowth helps gram-negative bacteria cells attach to other cells and surfaces. Without the help of pili, many pathogenic bacterial cells lose their ability to infect. This is because they find it hard to attach to the tissue of the host.

Moreso, some specialized pili are used for conjugation whereby two bacteria cells exchange fragments of plasmid DNA. There has been more interest in studying Pili as an organelle of adhesion as well as a vaccine component. This structure is the primary antigenic determinant, virulence factors, and impunity factors on the cell surface of a lot of gram-negative bacteria and some gram-positive bacteria.

Common gram negative bacteria representative species that possess pili include Enterobacteriaceae, Pseudomonadaceae, and Neisseriaceae. This organelle is responsible for virulence in some pathogenic gram-negative bacteria examples such as E. coli and Vibrio cholerae.

A bacterial cell having pili enhances the ability of the bacteria to bind to body tissues which in turn increases the replication rates of the bacterium and its ability to interact with the host organism. For bacteria species with multiple strains that have some pathogenic strains, it is likely that the non-pathogenic strains won’t possess pili while the pathogenic strains would.

Gram negative bacteria diagram

The image above is a gram negative bacteria labeled diagram, showing the structural components of the cell wall.

Types of gram negative bacteria

1. Gram negative cocci
2. Gram negative diplococci
3. Gram negative coccobacilli
4. Gram negative bacilli
5. Comma-shaped gram negative bacteria

After the gram staining test differentiates bacteria into gram-positive and gram-negative organisms, they are further classified into types. The gram-negative bacteria classification is based on the shape of gram negative bacteria. Gram negative bacteria shape could be spherical or rod-like, classifying them into the various types of gram-negative bacteria below:

Gram negative cocci

The gram-negative bacterium that has a spherical, ovoid, or generally round shape are grouped as gram-negative cocci. There are four examples of gram negative bacteria in this group that are of medical relevance. They include Neisseria gonorrhoeae, Neisseria meningitidis, Moraxella catarrhalis, and Haemophilus influenzae. N.gonorrhoeae causes sexually transmitted disease, N. meningitidis causes meningitis, whereas M. catarrhalis and H. influenzae cause respiratory symptoms.

Gram negative diplococci

This type of gram-negative bacteria includes round gram-negative bacterium that occurs in form of two joined cells. Typical examples of bacteria that are gram-negative diplococci include Neisseria spp and Moraxella catarrhalis.

The genus Neisseria includes more than 10 different species whereby a majority of them are gram negative and coccoid. The common gram negative bacteria representative species in this genus include Neisseria meningitidis, Neisseria cinerea, Neisseria polysaccharea, Neisseria gonorrhoeae, Neisseria oralis, Neisseria lactamica, Neisseria mucosa, and Neisseria subflava. Among these species listed, N. meningitidis and N.gonorrhoeae are the most pathogenic species.

The Neisseria gonorrhoeae is transmitted during copulation with an infected person. In women, this Neisseria species lodge in the reproductive tract tissue and cause cervical and uterine infections. Complications in pregnancy can arise if the membranous tissues within the fallopian tubes get infected. Moreso, this bacterium infects the urethra of men.

The N. meningitidis on the other hand is an example of gram-negative bacteria that can cause bacterial infections in or on the body. It can infect the lungs, nasopharynx, or skin. From the skin, the bacterium can enter the bloodstream and if extreme can result in fatality. This bacterium is also called meningococcus.

Among the genus Moraxellaceae, only one species, Moraxella catarrhalis has a morphology of diplococcus. The rest are gram-negative coccobacilli bacteria species. M. catarrhalis is a major pathogenic organism that contributes to infections in the human body. From studies, it has been seen that this bacterium tends to cause infection within the body. It can cause bacterial infections such as septic arthritis, pharyngitis, endocarditis, otitis, sinusitis, meningitis, bronchitis, and tracheitis.

Gram negative coccobacilli

One of the types of gram negative bacteria is the gram negative coccobacillus. These bacteria are gram negative bacteria with a shape that is intermediate between cocci (spherical bacteria) and bacilli (rod-shaped bacteria). They appear to be very short rods which may be mistaken for cocci.

Typical examples of gram negative bacteria in this group include Aggregatibacter actinomycetemcomitans, the genus Brucella, Haemophilus influenzae, Bordetella pertussis, Gardnerella vaginalis, Yersinia pestis, Coxiella burnetii, Haemophilus ducreyi, and Chlamydia trachomatis.

The coccobacillus, Aggregatibacter actinomycetemcomitans is an example of gram negative bacteria that is prevalent in subgingival plaques. Bordetella pertussis is a gram-negative coccobacillus that is known to cause whooping cough. Another gram-negative coccobacillus is Yersinia pestis, and Haemophilus ducreyi, which causes plaque and chancroid respectively. The genus Brucella is a medically important coccobacillus that causes brucellosis.

Moreso, the genus Moraxella is another example of gram negative bacteria with coccobacilli species. These species include Moraxella lacunata, Moraxella pluranimalium, Moraxella atlantae, Moraxella boevrei, Moraxella canis, Moraxella nonliquefaciens, Moraxella caprae, Moraxella lincolnii, Moraxella caviae,Moraxella osloensis, Moraxella cuniculi, Moraxella bovis, Moraxella equi, Moraxella ovis, and Moraxella saccharolytica.

Gram negative bacilli

These are the types of gram-negative bacteria that are shaped like rods. There are medically relevant bacteria species in this group that cause a lot of bacterial infections. Gram-negative bacillus such as Klebsiella pneumoniae, Legionella pneumophila, and Pseudomonas aeruginosa cause respiratory problems. Some are known to primarily cause urinary problems e.g Serratia marcescens, Escherichia coli, Enterobacter cloacae, and Proteus mirabilis. Moreso, gastrointestinal problems are caused by gram-negative bacilli like Salmonella enteritidis, Helicobacter pylori, and Salmonella typhi.

There are two large groups of gram negative bacilli that are responsible for the majority of clinical isolates. These groups include the family, Enterobacteriaceae, and the non-fermenters. The family Enterobacteriaceae is a large family of gram-negative bacteria that are bacilli. Enterobacteriaceae are a heterogeneous group that accounts for about 80% of gram-negative isolates with a myriad of pathogenic species in humans causing urinary tract infections, endotoxic shock, pneumonia, sepsis, diarrhea, meningitis, and many others.

Enterobacteriaceae are characterized to be non-sporulated bacilli that have variable motility and grow in the presence and absence of oxygen. They ferment organisms of glucose and are cytochrome oxidase negative. These bacterial species have the ability to reduce nitrate to nitrite. The family Enterobacteriaceae are one of the bile tolerant gram negative bacteria.

Another group of gram negative bacilli is the non fermenters. Compared to Enterobacteriaceae, the non-fermenter gram-negative bacilli (BNF) have a lower frequency of isolation. This group of bacteria causes severe, fatal infections, especially in the hospital environment. They can also cause opportunistic diseases in ICU patients who undergo invasive procedures.

The main bacilli non-fermenter microorganisms that cause disease in humans are Pseudomonas aeruginosa, Acinetobacter baumannii, Burkholderia cepacia, Stenotrophomonas., Alcaligenes, and Moraxella. They are aerobic and non-sporulated. Moreso, these bacteria species are incapable of fermenting sugars and using them through the oxidative route. They are highly resistant because they produce a variety of genes and have multiple mechanisms that are capable of mitigating microbicidal action.

Comma-shaped gram negative bacteria

This group of gram-negative bacteria includes bacteria species such as Campylobacter jejuni, Vibrio cholerae and Helicobacter pylori.

Gram negative bacteria chart

List of gram negative bacteria examples

1. Escherichia coli
2. Enterobacter cloacae
3. Serratia marcescens
4. Salmonella enteritidis
5. Salmonella typhi
6. Salmonella proteus
7. Shigella yersinia
8. Enterobacteriaceae
9. Pseudomonas aeruginosa
10. Moraxella spp
11. Helicobacter pylori
12. Stenotrophomonas
13. Campylobacter jejuni
14. Vibrio cholerae
15. Bdellovibrio
16. Acetic acid bacteria
17. Legionella pneumophila
18. Cyanobacteria
19. Klebsiella pneumoniae
20. Spirochaetes
21. Bordetella pertussis
22. Pasteurella
23. Proteus mirabilis
24. Green sulfur bacteria
25. Green non-sulfur bacteria
26. Brucella
27. Francisella tularensis
28. Neisseria gonorrhoeae
29. Neisseria meningitidis
30. Haemophilus influenzae
31. Acinetobacter baumannii
32. Citrobacter serratia

Above is a gram negative bacteria list of representative species including the pathogenic and non-pathogenic species.

Diseases

Bacteria can be found naturally around the body including the skin and intestines. When these bacteria are kept in balance, they work normally to help the body. Nevertheless, an infection can occur when there is an imbalance of the bacteria or the passing of the bacteria to where there are no bacteria in the body. If there are large amounts of these bacteria present in the body, or the immune system is weakened, an infection can develop.

Gram-negative bacteria are one of the most significant public health issues in the world because of their high resistance to antibiotics. These bacteria have great clinical relevance in hospitals because they usually cause patients to be in the intensive care unit (ICU). More so, the patients are usually at high risk of morbidity and mortality. Enterobacteriaceae and the non-fermenters are the two large groups responsible for most clinical isolates as well as other gram negative bacteria examples like Neisseria, Haemophilus spp., Helicobacter pylori, and Chlamydia trachomatis.

These bacteria have unique characteristics and can cause infections throughout the body. The bloodstream, lungs, nervous system, urinary tract, and soft tissues are the common sites that these bacteria infect. They are also known to infect surgical wounds. Hence, gram negative bacteria are associated with severe infections such as UTI (urinary tract infections), pneumonia, wound or surgical site infections, peritonitis (inflammation of the membrane that lines the abdominal cavity), bloodstream infections, and meningitis. Most importantly is the resistance of gram-negative bacteria to antibiotics which is currently on the rise.

List of gram negative bacteria and their diseases

	Common gram negative bacteria	Diseases
1	Escherichia coli	Food poisoning, urinary tract infections, gastroenteritis, newborn meningitis, normal colonic flora, diarrhea, sepsis, pneumonia, hemolytic-uremic syndrome, cystitis, and pyelonephritis
2	Pseudomonas aeruginosa	Lung and urinary tract infections
3	Francisella tularensis	Tularemia (similar to plague)
4	Legionella pneumophila	Legionnaire disease (atypical pneumonia), and Pontiac fever
5	Klebsiella pneumoniae	Pneumonia, meningitis, lung infection, UTIs, sepsis, and bloodstream infections
6	Salmonella typhi	Typhoid fever
7	Salmonella enteridis	Enterocolitis/gastroenteritis, septicemia, and osteomyelitis (sickle cell)
8	Brucella	Undulant fever (Brucellosis)
9	Acinetobacter baumannii	Several types of infections in wounded soldiers
10	Shigella dysenteriae	Shigellosis (dystentery)
11	Yersinia enterocolitica	Gastroenteritis
12	Neisseria gonorrhoeae	Gonorrhea (urethritis and PID), neonatal conjunctivitis, septic arthritis, and Fitz-Hugh-Curtis syndrome
13	Neisseria meningitidis	Meningococcemia, Waterhouse-Friderichsen syndrome, and meningitis
14	Haemophilius ducreyi	Chancroid (genital ulcers)
15	Serratia	UTI, wound infections, pneumonia, septicemia, and kidney stones
16	Enterobacteriaceae	Food poisoning, UTIs, lung, and bloodstream infections
17	Proteus	UTI, septicemia, and kidney stones
18	Campylobacter jejuni	Gastroenteritis, and Guillain-Barré syndrome
19	Vibrio cholerae	Cholera
20	Vibrio parahaemolyticus/vulnificus	Seafood diarrhea
21	Helicobacter pylori	Duodenal ulcers, chronic gastritis, gastric ulcers, and stomach cancer
22	Pseudomonas aeruginosa	Burn infection, endocarditis, pneumonia (CF), sepsis (nosocomial), external otitis, UTI (nosocomial), osteomyelitis, hot tub folliculitis, and corneal infections
23	Pasteurella multocida	Cellulitis/animal bite infection
24	Bacteroides fragilis	Normal gut flora, septicemia, peritonitis, abdominal abscesses, and wound infections
25	Bartonella henselae	Cat scratch fever, and bacillary angiomatosis
26	Haemophilus influenzae	Epiglottitis, meningitis (pediatric), otitis media, pneumonia (adult), and bronchitis
27	Yersinia pestis	Bubonic plague and pneumonic plague
28	Bordatella pertussis	Whooping cough
29	Moraxella catarrhalis	septic arthritis, pharyngitis, endocarditis, otitis, sinusitis, meningitis, bronchitis, and tracheitis.
30	Chlamydia trachomatis	Chlamydia (sexually transmitted infection)

Route of infection

The common ways at which gram negative bacteria are passed to vulnerable parts of the body include:

1. Open wounds
2. Usage of medical devices that pass into the body e.g urinary catheters or IV
3. Having contact with someone who has been infected or exposed to gram-negative bacteria

Risk factors

Hospitalization tends to be the most common risk factor for gram-negative bacterial infections. The longer a person stays in the hospital, the higher the risk of infection. Moreso, other factors can increase the risk of a bacterial infection. These risk factors include:

1. Dialysis for kidney disease, which involves the regular usage of IV lines or catheters
2. Recent surgery
3. Use of urinary catheter
4. War wounds
5. Usage of mechanical ventilator
6. Weakened immune system

Treatment

For gram negative bacteria, treatment is done with several classes of antibiotics. The antibiotics for gram negative bacteria are designed to target specific gram-negative bacteria and some also cover gram-positive bacteria. Infections caused by gram-negative bacteria may be treated initially with antibiotics that are effective against a wide range of infections. However, if the infection is not responding to the first antibiotics, the specific type of antibiotic may be changed.

Moreso, it takes time to get the bacterial test results because the bacteria need to grow in the lab. If a test result shows that a gram-negative bacteria is the causal organism, the doctor may change the initial antibiotic given.

Examples of gram negative bacteria antibiotics used for treatment include:

1. Aminopenicillins
2. Aminoglycosides
3. Monobactams (aztreonam)
4. Ciprofloxacin
5. Ceftriaxone
6. Ampicillin
7. Rifampin
8. Cephalosporin (3rd generation)
9. Ureidopenicillins
10. Beta-lactam-betalactamase inhibitor combinations (e.g. piperacillin-tazobactam)
11. Cefotaxime
12. Fluoroquinolones
13. Doxycycline
14. Metronidazole
15. Omeprazole
16. Folate antagonists
17. Quinolones
18. Carbapenems
19. Clarithromycin
20. Penicillin
21. Azithromycin
22. Erythromycin

Aminoglycosides, monobactams (aztreonam), and ciprofloxacin are the drugs that specifically target gram-negative bacteria. If gram-negative bacterial infections are left untreated, they can cause serious health complications and death.

Resistance

Gram-negative bacteria are protected by strong walls that make it difficult for many antibiotics to attack the bacteria. These bacteria may also have other defenses against antibiotic treatment such as:

1. The production of a chemical called Extended spectrum beta-lactamase (ESBL) that can stop certain antibiotics from working effectively.
2. Gram negative bacteria obtains genetic material from other bacteria to enable them resist antibiotics.

The current commonly used antibiotics may not work on gram-negative bacteria because they can learn to resist antibiotics. There is a type of gram-negative bacteria called multidrug resistant (MDR) gram negative bacteria that are resistant to multiple antibiotics. These MDR gram negative bacteria can cause bacterial infections that pose a severe threat for hospitalized patients, most especially ICU patients. Infection caused by multidrug resistant gram negative bacteria strains is correlated with increased mortality, morbidity, and prolonged hospitalization.

There are limited treatment options for these bacteria and so they pose a great threat to public health. Common factors that contribute to the existence and spread of MDR gram negative bacteria include:

• Overuse or misuse of existing antimicrobial agents: This has cause the development of adaptive resistance mechanisms by bacteria
• Lack of responsible antimicrobial stewardship: The use of multiple broad-spectrum agents has helped sustain the cycle of increasing resistance.
• The lack of good infection control practice.

Treating mdr gram negative bacteria has been challenging which has resulted in the revisit of old antibiotic drugs like colistin (Polymyxins) and fosfomycin. These antibiotics were traditionally considered to be toxic but have gained a major role in MDR gram negative bacteria treatment.

Why gram-negative bacteria are more harmful

One of the biggest differences (apart from shape) between these types of bacteria is that gram-negative bacteria are surrounded by a cell wall. This makes it more difficult for antibiotics to penetrate the barrier and kill the harmful bacterium living within.

The outer membrane surrounding gram-negative cells also prevents many bodily defense mechanisms (such as white blood cells) from effectively eliminating the infection.

Gram-negative bacteria are better at causing diseases, especially more serious illnesses, including pneumonia and blood infections. These types of infections are generally more difficult to treat with antibiotics because gram-negative bacteria are usually resistant to many common medications.

This is why gram-negative bacteria are considered more harmful than their positive counterparts.

Gram negative bacteria vs gram positive bacteria

The differences between gram-positive bacteria and gram-negative bacteria include the thickness of the peptidoglycan layer and the presence or absence of the outer lipid membrane. Gram-positive bacteria have a thicker peptidoglycan cell wall structure while gram-negative bacteria have a thin peptidoglycan cell wall. Due to this, the color of gram negative bacteria is pink or red in the gram stain test whereas gram-positive bacteria stain purple or blue.

The gram positive and gram-negative bacteria difference in color is based on their cell wall composition and the ability of the cell to retain the primary stain used in the Gram stain test. Molecules such as peptidoglycan, lipid, teichuronic acid, and teichoic acid are contained in the cell walls of gram-positive bacteria. This structural component is different from the cell walls of gram-negative bacteria. Gram negative bacteria cell wall is composed of peptidoglycan and an outer membrane. This outer membrane is made up of lipid, protein, and lipopolysaccharides.

The presence of an outer membrane in a gram-negative bacterium is also another major factor in the gram positive and gram negative bacteria difference. Gram-positive bacteria have just one membrane and are known as monoderm bacteria whereas gram-negative bacteria have two membranes and are known as diderm bacteria. These gram-negative bacteria tend to be more resistant to antibiotics than gram-positive bacteria because of their outer membrane. Gram-negative bacteria have a thin peptidoglycan layer and do not absorb any foreign material surrounding them.

Gram-positive bacteria, on the other hand, are more susceptible to antibiotics despite them having a thicker peptidoglycan layer because they lack an outer membrane. These bacteria are easier to kill because their thick peptidoglycan layer easily absorbs antibiotics and cleaning agents.

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