Microbiology: Gram Negative Bacilli

Question 1

Commonalities of GN bacilli

Answer 1

• All have LPS virulence factor
• Have 3 different serotypes: O-antigen (cell wall), K-antigen (capsular), H-antigen (flagellar)
• They cannot be ID’d from gram stain, must use culture on selective media, serotyping, and biochemical tests (i.e. presence of beta-galactosidase for utilization of lactose)
• Majority of infection cases
• Sort by site of infection, metabolism (aerobe vs. anaerobe), lactose fermentation, commensal, virulence
• Lactose fermentors: EEK (enterobacter, E coli, Klebsiella)
• Non-lactose fermenters (worse): PPYSS (pseudomonas, proteus, yersinia, salmonella, shigella

Question 2

Escherichia Coli

Answer 2

• Facultative anaerobe, lactose fermenter, many serotypes (some are commensal)
• Causes food borne illnesses (O157) and neonatal diarrhea (O155 and O111)
• Also causes gastroenteritis, UTI, neonatal meningitis, bacteremia
• 6 pathogenic subgroups (all end w/ “EC” for E coli): enterotoxigenic (ETEC), enteroinvasive (EIEC), enteropathogenic (EPEC), enteroaggregtive (EAEC), enterohemorrhagic (EHEC), and diffusely adherent (DAEC)

Question 3

Virulence factors of E coli

Answer 3

• Adhesins for attachment to epithelia
• Inhibition of immune response by K-Ag
• Toxins of many types

Question 4

Salmonella

Answer 4

• Many serogroups, not commensal, facultative anaerobe
• S typhi and S paratyphoid are exclusively human pathogens
• Transmitted through contaminated food and water

Question 5

Pathogenesis of salmonella

Answer 5

• Gastroenteritis by non-typhoid salmonella due to tissue invasion of epithelial lining of large and small bowel
• Penetration of lamina propria causes inflammatory diarrhea
• Typhoid fever caused by S typhi and S paratyphi, which enter circulation through M (phagocytic) cells of the gut (adhesins and invasins allow entry and replication)
• Multiple organs (liver, gall bladder, spleen) can be infected by bacteremia

Question 6

Virulence factors and Dx of salmonella

Answer 6

• Adhesins and invasins
• Survival after phagocytosis due to catalase and superoxide dismutase
• Acide tolerance response (ART) allows for survival of bacteria in stomach acid
• Dx based on growth on selective media and serotyping

Question 7

Vibrio cholerae

Answer 7

• Vibrionaceae appear as curved rods (fusiform), w/ single flagellum (motile)
• Aerobic
• Can survive high salt levels (halophilic)
• Can be obtained from fecal-contaminated food/water, or from some sea food

Question 8

Pathogenesis of cholera

Answer 8

• Severe, voluminous, watery diarrhea (rice water), no abdominal pain
• Cholera toxin (phage-borne AC binary toxin) disrupts H2O transport, net efflux
• Has adhesion factors (mucinase), and siderophore (Fe binding protein)
• Dx is culture, with biochemical and serological testing (non-lactose fermenter, oxidase positive

Question 9

Pseudomonadaceae

Answer 9

• Obligate aerobe, prototrophic (can use inorganic matter)
• Oxidase positive but does not reduce nitrates
• Ubiquitous, resists disinfection
• Associates w/ nosocomial (hospital-acquired) infections

Question 10

Pseudomonas aeruginosa

Answer 10

• Obligate aerobe, flagellated, w/ capsule and blue-green pigment
• Produces many nosocomial infections: bronchitis, bronchopneumonia, UTI, bacteremia, endocarditis, complications of burn and surgery wounds
• Can also come from community: complications of cystic fibrosis, ear infections, skin infections (wounds, folliculitis)

Question 11

Virulence factors and pathogenesis of P aeruginosa

Answer 11

• Adhesin (pili and neuraminidase), flagellum
• Toxins (exotoxin A is similar to diphtheria toxin (inhibits protein synthesis)
• Pigments turn puss green-blue
• Exonzs (elastase, phospholipase, ect)
• Biofilm formation, especially in lungs

Question 12

Dx of P aeruginosa

Answer 12

• Culture, non-lactose fermenter

- Characteristic blue/green pigment

Question 13

Haemophilus influenza

Answer 13

• Small pleiomorphic rods, facultative anaerobe
• Commensals: H influenza, H parainfluenza, H ducreyi (STI)
• Disease comes from endogenous bacteria

Question 14

Pathogenesis of H influenza

Answer 14

• Can cause meningitis (encapsulated)
• Also otitis media, sinusitis, epiglottitis, pneumonia (non encapsulated)
• Virulence associated w/ acquired capsule (serotypes a-f)
• Serotype b is most pathogenic
• Other virulence factors: LPS endotoxin, possibly PRP (polyribitol phosphate, antiphagocytic and acquired), and IgA protease to counter mucosal immunity
• Dx is through typing of X (protophorphyrin) and V (NAD+/NADP+) requirement (only H influenza needs both to survive)