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
16
Q
CLINICAL SIGNIFICANCE Shigella
A
- causes classic bacillary dysentery - shigellosis only in higher primates
- spread from human to human through faecal-oral route
- reservoir - carrier who sheds organism in faeces (carrier state lasts for 1 to 4 weeks)
- from carriers organisms spread by flies, fingers, water, food
- high communicability attributed to low infective dose needed to produce disease (10 to 100 organisms)
- characterised by high fever, chills, convulsions, abdominal cramps, tenesmus, frequent stools (small-volume stools with blood and mucus), generalised myalgias
- in previously healthy adults spontaneous cure occurs in 2 to 7 days, mortality due to dehydration and electrolyte
imbalance
17
Q
GENERAL CHARACTERISTICS AND
NATURAL HABITAT Salmonella
A
- all strains motile by peritrichous flagella with exception of Salmonella Gallinarum and its variant Pullorum
- frequently found in sewage, river, sea-water, in certain foods
- primary pathogens of lower animals
- humans only known reservoir for Salmonella Typhi
- animals principal source of non-typhoidal Salmonella infections in humans
18
Q
Antigenic structure Salmonella
A
- O antigens −somatic antigens, group specific − heat-stable polysaccharides, form part of cell wall
lipopolysaccharide - Vi antigen
−surface heat-labile polysaccharides −inhibit agglutinability of organisms by homologous O antisera - H antigens − flagella antigens, type specific − heat-labile proteins of flagellae − have unique character of diphasic variation
- Phase 1 - specific phase
- Phase 2 - non-specific phase
19
Q
THE KAUFFMANN-WHITE DIAGNOSTIC
SCHEME
A
- used for serologic classification of Salmonella
20
Q
VIRULENCE FACTORS Salmonella
A
- O antigenic side chains and Vi antigen
− responsible for ability of organism to attach to host receptor cells and to survive intracellularly − prevent activation and subsequent deposition of complement factors on bacterial surfaces − protect organism against phagocytosis - invasiveness
− like shigellae virulent salmonellae penetrate epithelial lining of small bowel
− unlike shigellae, salmonellae do not merely reside in epithelial lining but penetrate into subepithelial tissue - endotoxin
− responsible for fever seen during bacteraemic stages of enteric fever (releases endogenous pyrogens from leukocytes) - enterotoxins − similar to heat-labile - LT and heat-stable - ST enterotoxins of Escherichia coli
- cytotoxin
− important in cellular invasion and cellular destruction
21
Q
CLINICAL SIGNIFICANCE -
SALMONELLA TYPHI
A
- causes typhoid fever
- human carriers represent sole source of organisms
- carriers can be: − convalescents who excrete organism for short period of time− chronic carriers who shed organism for longer than 1 year
- 3% of all patients become chronic carriers, majority older women with gallbladder disease
- individuals infected by ingesting food or water contaminated by carrier
22
Q
CLINICAL SIGNIFICANCE - OTHER
SALMONELLAE
A
- any serotype can cause salmonellosis
- in contrast to typhoid fever, contaminated animals and animal products major sources of other salmonellae
- poultry and beef products represent largest sources
- human carrier serves as source of secondary infections
- patients with gastroenteritis shed organism for several weeks
- contaminated food and water vehicles of transmission
- Gastroenteritis
− represents actual infection of colon
− occurs 18 to 24 hours after ingestion of organism− characterised by: diarrhoea, fever, abdominal pain - Septicaemia
− characterised by: fever, chills, anorexia, anaemia
23
Q
Typhoid and enteric fevers Salmonella
A
- Typhoid fever and other enteric fevers
− most severe enteric fever caused by serotype Typhi
−serotypes Paratyphi A and Paratyphi B cause enteric
fevers with milder symptoms and lower mortality
− during 1st week of infection symptoms of lethargy,
fever, malaise, general aches and pains
−can be confused with variety of other illnesses
−constipation rather than diarrhoea present
− during 2nd week symptoms of bacteraemia present
−infection of biliary tree and other organs occurs
− patient severely ill with sustained fever, often
delirious
−abdomen tender, may have typical rose
-coloured spots, diarrhoea begins
−complications include: intestinal perforations,
bleeding, pneumonia, abscess formation
− death rate ranges from 2% to 10%
24
Q
COLLECTION, TRANSPORT AND
STORAGE OF SPECIMENS Enteric pathogens
A
specimen of choice: freshly passed stool
* Cary-Blair transport medium preserves best viability of intestinal pathogens, including Campylobacter spp., Vibrio spp.
* blood - best specimen for detection of septicaemia and enteric fevers during 1st week of illness
* 80% of typhoid fever patients have positive blood cultures during 1st week of illness, with only 25% of patients having positive blood cultures by 4th week
* faecal specimens positive in only 25% of typhoid fever patients during 1st week, by 3rd week faeces positive in 85% of all patients
* urine and other specimens (sputum, spinal fluid) appropriate for diagnosis of complications of Salmonella infections
25
CULTURE MEDIA - ENRICHMENT MEDIA Enteric pathogens
* Selenite F broth
− designed for enrichment of salmonellae, including
Salmonella Typhi
* Cold-temperature enrichment
− used to increase isolation of Yersinia enterocolitica
−incubation at 4oC for 3 weeks in phosphate-buffered saline before subculture permits survival and growth of this organism, while other bacterial species inhibited
26
CULTURE MEDIA - PLATING MEDIA Enteric pathogens
Non-inhibitory media
* 5% sheep blood agar medium
Differential media with little selectivity
* MacConkey agar medium (MC)
− bile salts and crystal violet inhibit growth of gram-positive bacteria and some fastidious gram-negative bacteria
* Eosin methylene blue agar medium (EMB)
− eosin and methylene blue inhibit gram-positive and fastidious gram-negative bacteria
Differential media with moderate selectivity
* Hektoen enteric agar medium (HE)
− high bile salt concentration inhibits growth of all gram-positive bacteria, retards growth of many strains of coliforms
* Xylose-lysine-deoxycholate agar medium (XLD)
− bile salts in relatively low concentration make this medium less
inhibitory to growth of coliform bacilli
Special media
* Sorbitol-MacConkey agar medium (SMAC) − contains 1% D-sorbitol instead of lactose − for detection of verotoxin-producing Escherichia coli serotype O157:H7
* Cefsulodin-irgasan-novobiocin agar medium (CIN)
− selective-differential medium for detection of Yersinia enterocolitica
27
INCUBATION CONDITIONS Enteric pathogens
* all media incubated:
− at 35oC to 37oC
− for 24 to 48 hours
− in ambient air
* CIN agar medium incubated:
− at room temperature
− for 48 hours
− in ambient air
28
IDENTIFICATION of enteric pathogens
* Colony morphology
* Gram stain of colony
* Oxidase Test
* Nitrate reduction Test
* Biochemical Tests
* MALDI-TOF MS
* Serologic Characterisation
29
SEROLOGIC CHARACTERISATION enteric pathogens
* by performing slide agglutination test
Escherichia coli
* based on determination of O and H antigens
Yersinia enterocolitica
* based on determination of O antigens using monovalent O antisera
Shigella spp.
* based on determination of O antigens
Salmonella spp.
* based on determination of O antigens, H antigens, and for Salmonella Typhi also on Vi antigen
30
ANTIBIOTIC SUSCEPTIBILITIES Enteric pathogens
Shigella spp.
* ampicillin or amoxicillin
- drug of choice for sensitive isolates
Salmonella species
* ampicillin or chloramphenicol
- drug of choice in cases of enteric fever or septicaemia
* ampicillin
- drug of choice for treatment of chronic carriers of Typhi serotype
