Week 1 Flashcards
1
Q
ENTERIC PATHOGENS general characteristics
A
- Gram-negative rods
- motile by peritrichous flagella or non-motile
- do not form spores, capsulated or non-capsulated
- aerobic and facultatively anaerobic
- oxidase negative, catalase positive
- reduce nitrate to nitrite
- ferment (rather than oxidise) D-glucose, often with gas production
2
Q
GENERAL CHARACTERISTICS AND
NATURAL HABITAT E. coli
A
- widely distributed on plants, in soil, water, intestines of animals and humans
- most prevalent facultative gram-negative rod in faeces
- most common bacterium isolated in clinical microbiology laboratories
- most common cause of urinary tract infection
- common cause of intestinal and extraintestinal infections
3
Q
ANTIGENIC STRUCTURE of E. coli
A
- serologic typing based on determination of:
− O antigen type (more than 170 O antigens)
− H antigen type (75 H antigens)
− K antigen type (100 K antigens)
4
Q
VIRULENCE FACTORS - SURFACE
FACTORS E. coli
A
- polysialic acid capsule - K1
−enables organism to resist killing by neutrophils and serum - fimbriae or pili
−enable organism to attach to various host tissues - adhesins
−important in uropathogenicity and intestinal infections - O antigens
−important in binding of organism to host cells
− protect organism from effect of complement - lipid A
− portion of lipopolysaccharide - LPS
−responsible for endotoxic effects seen in bacteraemic patients
5
Q
ENTEROTOXINS E. coli
A
- heat-labile enterotoxin - LT
− similar to enterotoxin of Vibrio cholerae
− stimulates adenyl cyclase activity in epithelial cells of small intestinal mucosa resulting in loss of fluids and electrolytes
− genetic information encoded on plasmids - LT-II − second class of LT
− contains at least 2 different toxins LT-IIa and LT-IIb
− mode of action similar to that of LT
− genetic information encoded on bacterial chromosome - heat-stable enterotoxins - ST
− STa or STI - methanol soluble - active in suckling mice and neonatal pigs
- activates guanylate cyclase in intestinal mucosa cells
causing secretory response
− STb or STII - not methanol soluble - active only in weaned pigs
- mechanism of action unknown
verocytotoxins (Shiga toxins) - VT1 and VT2 (Stx1 and
Stx2)
−termed verocytotoxins because of their irreversible
cytotoxic effect on Vero tissue culture cells
−inhibit protein synthesis in eukaryotic cells
6
Q
VIRULENCE FACTORS - OTHER FACTORS E. coli
A
- filterable haemolysin
− lyses erythrocytes of variety of species − cytotoxic for leukocytes, chicken and mouse fibroblasts − contributes to inflammation, tissue injury, impaired host defences− encoded by chromosomal genes or by plasmid - cytotoxic necrotising factor - CNF
− protein
− causes necrosis in rabbit skin
− produced only by haemolytic strains
7
Q
CLINICAL SIGNIFICANCE E. coli
A
- leading cause of nosocomial and community-acquired urinary tract infections
- causes pulmonary infections
- important cause of neonatal meningitis, rarely cause of meningitis in older populations
- mortality rate between 40% and 80%, majority of survivors have subsequent neurological or developmental abnormalities
- can cause wound infections, particularly in abdomen
- peritonitis frequent complication of ruptured appendices
- can invade bloodstream from any of primary infection sites
8
Q
Enterotoxigenic E. coli (ETEC)
A
- causes: dehydrating infantile diarrhoea in developing countries, traveller’s diarrhoea
- can produce nausea, abdominal cramps, low fever, profuse watery diarrhoea -like mild case of cholera
- traveller’s diarrhoea can be severe
9
Q
Enteropathogenic E. coli (EPEC)
A
- causes enteric diseases particularly in small children
- characterised by fever, vomiting, prominent and watery diarrhoea with mucus but not blood
10
Q
Enteroinvasive E. coli (EIEC)
A
- produces enteritis characterised by fever, abdominal cramps, watery diarrhoea, or typical bacillary dysentery with leukocytes, blood, mucus like Shigella spp.
- like Shigella spp. and unlike most E. coli strains associated with enteritis, invades colonic mucosa, multiplies within mucosal epithelial cells, disrupts epithelial cells
- most strains non-motile, late- or non-lactose-fermenters
11
Q
Enterohaemorrhagic E. coli (EHEC)
A
- causes asymptomatic infection, mild diarrhoea, diarrhoeal illness characterised by non-bloody progressing to bloody diarrhoea, abdominal cramps, lack of significant fever
- illness usually self-limited, lasts 5 to 8 days
- 2% to 7% of patients develop haemolytic uraemic syndrome - HUS* strains water-borne and food-borne, person-to-person transmission occurs by faecal-oral route
- more than 50 serotypes, E. coli O157:H7 prototype
- does not ferment D-sorbitol - characteristic used to differentiate this serotype on MacConkey-sorbitol agar medium (SMAC)
12
Q
Enteroaggregative E. coli (EAEC)
A
- associated with chronic diarrhoea in many parts of world
- in children produces watery, mucoid diarrhoeal illness with low-grade fever, little or no vomiting
13
Q
Diffusely adherent E. coli (DAEC)
A
- majority of patients have watery diarrhoea without blood or
faecal leukocytes - carries gene encoding surface fimbria designated F1845* pattern of adherence to epithelial cells in vitro: diffuse
adherence
14
Q
NATURAL HABITAT AND ANTIGENIC
STRUCTURE Shigella
A
- humans reservoir only, not normal flora
- cause bacillary dysentery
- Shigella divided into 4 major O antigenic groups: A, B, C, D
- in addition to major O antigens, groups A, B, and C contain minor O antigens
- group D biochemically distinct from other Shigella spp.
- some strains posses K or envelope antigens (not important for serologic typing)
- since all Shigella spp. non-motile - no H antigens
15
Q
VIRULENCE FACTORS Shigella
A
- O antigens −2 types:
- smooth lipopolysaccharide structure termed phase I colony type
- rough lipopolysaccharide structure termed phase
II colony type
−responsible for ability to survive passage through host defenses - Invasiveness
−virulent strains penetrate mucosa and epithelial cells of
colon - Shiga toxin
−interferes with protein synthesis − mode of action as toxic plant protein ricin
− neurotoxic −cytotoxic −enterotoxic −causes microvascular damage to intestine resulting in haemorrhage
