Medically Important Gram Negative Bacteria

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Bacteria cell wall is composed of

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Plasma membrane + Peptidoglycan layer

Peptidoglycan layer is made of alternating N-Acetylmuramic acid (NAMA) and N-acetylglucosamine (NAG) residues, where pentapeptide crosslinks attach NAMA of one layer to NAG of the next

Very thick in Gram positive, thin in Gram negative

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Outer membrane

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* Only found in Gram negative bacteria
* Helps to reduce permeability to antibiotics

* Composed of lipids, polysaccharides, lipoproteins and lipopolysaccharides

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Lipopolysaccharides

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are endotoxins, have three parts…

* O antigen (O polysaccharide or O-specific side chain)
* Core oligosaccharide

* lipid A

presence of endotoxins in blood- endotoxemia

* This can lead to septic shock, causing hypotension, hypoxia, microvascular abnormalities and multiple organ failure
* Also activates the cytokine cascade, causing increased levels of interleukins and tumour necrosis factor

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Capsule

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* A polysaccharide layer that lies outside the cell envelope of the bacteria
* A virulence factor (e.g. prevents phagocytosis)

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Pili / Flagella

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* Pili are hairlike appendages found on the surface of many bacteria and can transfer DNA or help bacteria move
* Flagella aids movement, and can function as a sensory organ

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Methods of classifying / identifying bacteria

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* Staining properties
* Morphology (e.g. cocci, bacilli (rods), coccobacilli, spirals)
* Dependence on oxygen (strictly aerobic; strictly anaerobic; facultatively anaerobic; microaerophilic)
* Physiological tests (e.g. motility, optimal growth temperature)
* Biochemical tests (e.g. oxidase, sugar fermentation reactions)
* Profiling of cellular proteins by mass spectrometry
* Phylogenetic studies

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* Identifying and understanding bacteria is important as it allows…

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* Understanding of the clinical significance of an isolate
* A prediction of the most likely pathogen in a patient
* The most appropriate antimicrobial therapy to be chosen
* A prediction of the most likely sources, routes and sites of infection

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Enterobacteriaceae

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Normally resides in large bowels

Facultative anaerobic Gram negative bacilli

Oxidase negative, positive for glucose fermentation

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Examples of enterobacteriaceae

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* Escherichia
* Klebsiella
* Proteus
* Providencia
* Morganella
* Salmonella
* Shigella,
* Enterobacter
* Citrobacter
* Serratia

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* Major surface antigens of Enterobacteriaceae include…

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* O antigen (somatic LPS antigen)
* H antigen (flagellar antigen)
* K antigen (capsular antigen)

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* Diseases caused by Enterobacteriaceae include…

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* Gastrointestinal infections, e.g. Salmonella, Shigella, some Escherichia coli strains
* Extra-intestinal infections: e.g. urinary tract infection, bacteraemia, meningitis by various bacteria

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* Escherichia coli

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* The most common commensal Enterobacteriaceae and one of the most common Gram negative bacteria causing human infections
* Causes intestinal disease: e.g. traveller’s diarrhoea, haemorrhagic colitis (E. coli O157:H7 and other serotypes carrying the specific toxin)
* Causes extra-intestinal disease: e.g. urinary tract infection, meningitis in neonates, bacteraemia, intra-abdominal infections…

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* Salmonella

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* Salmonella enterica is one species but over 2500 serotypes exist, each distinguished by different O and H antigen compositions
* Generally classified into typhoidal and nontyphoidal salmonellae
* They are important causes of foodborne (and sometimes waterborne) infections
* Gastroenteritis, the most common manifestation
* Typhoid and paratyphoid fevers: a systemic infection caused by Salmonella Typhi and Paratyphi, infects humans only
* Disseminated or focal infections, e.g. osteomyelitis, meningitis, abscess formation

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Shigella

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* Species include Shigella dysenteriae, S. flexneri, S. boydii, S. sonnei
* Causes bacillary dysentery (blood and mucus in stool), can be fatal (especially in children)
* Transmitted by contaminated food or water or human-to-human through the faecal-oral route
* Pathogenic mechanism is mainly through production of the Shiga toxin (which inhibits protein synthesis in target cells)

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* Klebsiella

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* The most common species causing human infections is Klebsiella pneumoniae
* Frequently causes pneumonia, urinary tract infection, abscesses (e.g. liver abscess)

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* Proteus, Providencia, Morganella

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* Urease positive
* Urinary tract infection, other hospital-acquired infections

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* Citrobacter, Enterobacter, Serratia

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* Often associated with hospital-acquired infections and resistant to many antibiotics

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* Vibrionaceae and related genera

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* A family of curved Gram negative rods, oxidase positive, positive for glucose fermentation, actively motile by polar flagellum
* Facultative anaerobes
* Widely distributed in natural water bodies, seawater and fresh water.
* Usually cause gastrointestinal tract infections but occasionally cause severe systemic infections
* Aeromonas and Plesiomonas are similar to Vibrio species in many aspects, yet not currently classified in the family Vibrionaceae

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Vibrio cholerae

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* Causes cholera or milder forms of gastroenteritis
* Very profuse watery diarrhoea produced(rice water stool), can be rapidly fatal due to dehydration
* Serotypes (based on O antigen) are Vibrio cholerae O1 Vibrio cholerae non-O1, non-O139 Vibrio cholerae O139
* Increasing intracellular cAMP at intestinal epithelium
* Inhibiting sodium and chloride absorption
* Increasing chloride ion and water secretion

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Other non-cholera vibrios

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* V. parahaemolyticus (a very common cause of bacterial gastroenteritis)
* V. vulnificus (can cause a severe form of soft tissue infection called necrotizing fasciitis)

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* Pseudomonas and other non-fermenters

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* These are aerobic Gram negative bacilli that do not ferment glucose.
* Most are saprophytes and widely found in soil, water, and other moist environment
* Can sometimes colonise the skin
* e.g. Pseudomonas aeruginosa, Acinetobacter baumannii, Stenotrophomonas maltophilia

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Pseudomonas aeruginosa

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* Produces a green pigment and a characteristic odour
* Often resistant to multiple antibiotics and even disinfectants

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* Bordetella pertussis

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* The cause of pertussis (whooping cough)

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* Campylobacter

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* Spiral-shaped Gram negative rods. Microaerophilic
* Widely distributed in nature in animals including livestock
* Human infections usually occur as a result of ingesting contaminated animal products
* They are one of the most common bacterial causes of acute infective diarrhoea
* Can also cause systemic infections such as bacteraemia

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* Helicobacter pylori

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* Causes duodenal ulcer, gastric ulcer, gastric cancer, gastric lymphoma

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* Haemophilus

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* Short Gram negative coccobacilli
* X factor (haemin).
* V factor (NAD nicotinamide adenine dinucleotide or NADP nicotinamide adenine dinucleotide phosphate)
* e.g. Haemophilus influenzae

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Haemophilus influenzae

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* Causes meningitis, pneumonia, sinusitis, otitis media, and other invasive infections (but despite its name, not the cause of influenza)
* Possesses a polysaccharide capsule, six types from a to f, and non-typeable
* Type b causes most cases of invasive disease in children and infants
* Vaccines against type b capsular polysaccharide are available

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* Brucella

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* Short Gram negative coccobacilli
* Small non-motile, non-sporing, coccobacilli
* Brucellosis is the disease caused by Brucella species, is a zoonosis which is highly infective, causes muscular pain and sweating
* Main animal hosts are sheep and goat (B. melitensis), cattle (B. abortus), pig (B. suis)

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* Legionella

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* Environmental Gram negative bacilli
* Most common human pathogen is L. pneumophila, causes Legionnaires’ disease

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* Spirochaetes

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* Spiral-shaped organisms
* Usually not readily visualised by routine light microscopy
* Examples include Treponema, Borrelia, Leptospira

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* Treponema pallidum

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* Not cultivable in vitro.
* Causes syphilis, a sexually-transmitted infection.
* Without treatment, the organism can persist in the body for decades, resulting in chronic infections of the central nervous system, cardiovascular system…
* Can also cause congenital infection of the foetus and infant if the infection in the pregnant woman is active and untreated

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* Leptospira interrogans

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* Corkscrew-shaped bacteria, which differs from other spirochaetes by the presence of end hooks
* Leptospirosis is a zoonosis (transmitted from vertebrate animals to humans), occurs worldwide but is commonest in tropical and subtropical areas with high rainfall
* The disease is found mainly in situations when humans come into contact with the urine of infected animals or a urine-polluted environment