MICROBIOLOGY LECTURE 10 - Enterobacteriaceae, Shigella, SalmonellaNotes from LectureUSTMED ’07 Sec C AsM

ENTEROBACTERIACEAEEnterobacteriaceae General Characteristics1. ubiquitous organisms that are found in the soil, water,

decaying matter and the large intestines of animal and insects

2. natural habitat in humans – gastrointestinal tract referred to as “enteric bacilli” or “enterics”

3. constitutes only a minor fraction of the total microbial flora of the gastrointestinal tract

4. includes some of the most important cause of gastrointestinal disease

a. salmonellae – typhoid feverb. shigella – bacillary dysentery5. some are members of the normal commensal flora that

can cause opportunistic infectionsa. Escherichia colib. Klebsiella pneumoniac. Proteus mirabilis6. Responsible for the majority of nosocomial (hospital-

acquired) infections7. causes disease

a. in the gastrointestinal tracti. Shigella

b. Outside the gastrointestinal tracti. Klebsiellaii. Enterobacteriii. Serratiaiv. Proteus

c. Both inside and outside the gastrointestinal tract

i. Escherichia coliii. Salmonellaiii. Yersinia

8. Infection may originate from the:a. Animal reservoir – most Salmonella infectionsb. Human carrier – Shigella and Salmonella typhic. Endogenous spread of organisms in

susceptible patient – Escherichia coli9. Possesses complex antigenic structure10. Produces a variety of toxins and other virulence factors

Classification of Enterobacteriaceae is based on:1. DNA homology2. Biochemical properties3. Serologic reactions4. Susceptibility to genus specific and species

specific bacteriophages5. Antibiotic susceptibility pattern

Edwards and Ewing Classification of Enterobacteriaceae

Tribe Genus SpeciesI. Escherichiae I. Escherichia

II. Shigella

Eschericia coli

Shigella dysenteriaeShigella flexneriShigella boydiiShigella sonnei

II. Edwardsiellae

I. Edwardsiella

Edwardsiella tarda

III. Salmonellae I. Salmonella

II. Arizona

III. Citrobacter

Salmonella choleraesuisSalmonella typhiSalmonella enteritidis

Arizona hinshawiiCitrobacter freundii

Citrobacter diversus

IV. Klebsiella I. Klebsiella

II. Enterobacter

III. Seratia

Klebsiella pneumoniaeKlebsiella ozaenaeKlebsiella rhinoscleromatis

Enterobacter cloacaeEnterobacter aerogenesEnterobacter hafniaEnterobacter agglomerans

Serratia marcescensSerratia liquefasciensSerratia rubidaea

V. Proteae I. Proteus Proteus vulgarisProteus mirabilisProteus morganiiProteus rettgeri

II. Providencia Providencia stuartiiProvidencia alcalifaciens

VI. Yersineae I. Yersinia Yersinia enterocoliticaYersinia pseudotuberculosisYersinia pestis

VII. Erwiniae (plant pathogens)

I. Erwinia

Microscopic Morphology1. small (0.5 by 3.0 um), gram negative non-

sporeforming rods2. motile by means of peritrichous flagella; non-

motile – Shigella and Klebsiella3. well-defined capsule

Klebsiella Enterobacter Serratia Some Escherichia strains- slime layer – loose ill defined coating

4. fimbriae or pili – responsible for attachment of the bacterial cells to other bacteria, host cells and bacteriophage

5. cell wall consists of:a. mureinb. lipoproteinc. phospholipidsd. proteine. lipopolysaccharide – contains the specific

polysaccharide side chaini. determines the antigenicity of

the various speciesii. portion of the cell responsible for

the endotoxic activity

- 80% of the cell wall is joined to the lipid of the lipoprotein to form a lipid bilayer

Colonial Morphology1. Similar colonial morphology in blood agar plate

a. Moist, smooth, gray coloniesb. Some strains are beta hemolytic

2. Eschericia coli – convex, cicular, smooth with distinct edges

3. Enterobacter – similar but somewhat more mucoid4. Klebsiella – large and very mucoid and tend to

coalesce with prolonged incubation5. Salmonella and Shigella – similar to Escherichia

coli but do not ferment lactose6. Pigmented genera

Serratia – reddish orange Edwardsiella – yellowish white

Enterobacteriaceae on 5% sheep blood agar. Characteristic colonial morphology on 5% sheep blood agar showing large, dull, grey, nonhemolytic colonies. Hemolysis is variable and not characteristic of any one genus.

20% of the cell wall; responsible for cellular rigidity

Klebsiella pneumoniae on MacConkey agar. Rapid lactose fermenting colonies of Klebsiella pneumoniae appears pink, large, glistening, and mucoid. This strain is probably encapsulated and therefore appears mucoid. Although this appearance is associated with Klebsiella pneumoniae. It is not unique for

that species.

Pigmented Serratia sp. on MacConkey agar. These colonies appear red and should not be confused with the pink color due to lactose fermentation shown in 8.3. Rare strains of Serratia spp. produce pigment, which is seen on all solid media including the blood agar plate.

Biochemical Characteristics1. facultatively anaerobic or in low oxygen

atmosphere ferments carbohydrates2. with sufficient oxygen – utilize the tricarboxylic

acid cycle and the electron transport system for energy production

Enterobacteriaceae PseudomonadaceaeAll are facultative anaerobes Strict aerobesAll ferment glucose with acid or acid and gas

Does not ferment glucose

Oxidase negative Oxidase positiveReduce nitrates to nitrites Does not reduce nitrates

to nitrites

Escherichia1. positive indole2. positive lysine decarboxylation3. positive mannitol fermentation4. produce gas from glucose5. urine isolate – identified by:

a. hemolysis in blood agarb. typical colonial morphology in EMB –

greenish metallic sheen or iridescent sheen

c. positive spot indole testd. positive β-glucoronidase using the

substrate 4-methylumbelliferyl-β-glucoronide (MUG)

e. negative oxidase test

Klebsiella-Enterobacter-Serratia Group

1. Klebsiellaa. mucoid growthb. large polysaccharide capsulec. lack of motilityd. positive lysine decarboxylase and citrate

2. Enterobactera. positive tests for motilityb. positive citratec. positive ornithine decarboxylased. produce gas from glucosee. enterobacter agglomerans – small

capsule3. Serratia

- produces DNase, lipase and gelatinase- all are positive for Voges Proskauer test

Proteus-Morganella-Providencia Group- deaminate phenylalanine- motile- grow on KCN medium- ferment xylose

1. Proteus- swarming on solid media – actively motile by

means of peritrichous flagella2. proteus species and Morganella morganii

– urease positive3. Providencia

- urease negative4. Proteus-Providencia

– ferment lactose very slowly or not at all5. Proteus mirabilis

– more susceptible to antimicrobial drugs including penicillin

Citrobacter- citrate positive- differs from salmonella – (-) lysine

decarboxylation- ferments lactose very slowly

Shigella- nonmotile- nonlactose fermenter- ferments other carbohydrates producing acid

but not gas- closely related to Escherichia coli- share common antigen with one another and

with other enteric bacteria

Salmonella- motile- ferments glucose and mannose without gas

but do not ferment lactose or sucrose- produces H2S

Proteus species on 5% sheep blood agar. Growth appears to spread as a film on the plate from the original colony or streak line, often extending in waves. This characteristic of Proteus spp. is called swarming and suggests that the microorganism is motile by means of flagella.

Antigenic Structure1. O-antigens – somatic – heat stable

a. species specific polysaccharide which makes up a part of the LPS component of the cell wall

b. endotoxic activity expressed by the LPS complex reside in the lipid A molecule

2. K or capsular antigen – heat labilea. a polysaccharide component found in some

members of the enteric bacillib. called Vi antigen in Salmonellac. basis of serologic typing of Klebsiellad. cells possessing K antigen are more

pathogenic than those that lack theme. inhibits phagocytosis and the effects of serum

antibody3. H antigens or flagellar antigens – heat labile

a. protein found only in motile formsb. motile bacteria may lose their capacity to

produce flagella but retain their O antigen specificity

- Other antigens4. Enterobacterial common antigen or kunin antigen

a. haptenic substance occurring in two aggregative forms

o linked to the lipopolysaccharide and is immunogenic

o linked to a carrier molecule and nonimmunogenic

b. LPS linked ECA rare but found in Escherichia coli 014

c. Role in virulence and pathogenesis not established

d. Chemical nature undefined5. Fimbrial antigens

a. believed to be virulence factors because of their adhesive properties

b. protein in nature and present in all enteric bacilli

Determinants of Pathogenicity1. Endotoxin

a. LPS of the cell wall – endotoxin – associated with the bacterial cell and is toxic to animals

b. Toxicity of LPS resides in the lipid A moleculec. Effects when injected to animals

- fever- fatal shock- leukocyte alterations- regression of tumors- alterations in host response to infection- Sanarelli-Shwartzman reaction- Various metabolic changes

d. cellular targets are variede. exact mechanism of action – not clearly delineatedf. 30% of enteric bacteremia will develop endotoxic

shock chief defect – pooling of blood in the microcirculation – causes cellular hypoxia and metabolic failure due to inadequacy of blood in vital organs

2. Enterotoxinsa. toxins that usually affect the small intestine

causing a transduction of fluid into the intestinal lumen and subsequent diarrhea

o salmonellao shigellao strains of Escherichia coli, Klebsiella

pneumoniaeo citrobacter freundiio Enterobacter

b. enterotoxin producing Escherichia coli – major causes of travelers diarrhea and diarrhea in developing countries

c. incidence of disease cause by enterotoxin producing Citrobacter, Klebsiella and Enterobacter species – unknown

d. role of enterotoxins in salmonellosis and shigellosis – unclear; tissue penetration important in pathology

3. Shiga toxins and Shigalike toxins (verotoxins)a. Shiga toxin – interferes with protein synthesis of

mammalian cells; role in shigellos is unclearb. Certain E. coli strains – produce similar toxins

called verotoxins because of their action of Vero (African green monkey) tissue culture cells; important causes of hemolytic diarrhea and hemolytic uremic syndrome

4. Colonization factorsa. capsule of Klebsiella pneumoniae – to prevent

phagocytosis

b. Vi antigen of Salmonella typhi – prevents intracellular destruction of the bacterial cell

c. Fimbriae such as the CFA of human isolates – necessary for the attachment of the organism to target tissues

d. O antigen – may bind the organism to certain tissue receptor sites

Clinical Infection

Types of infection1. leading cause of bacteremia and urinary tract

infections2. can invade any body site and can cause wound

infections, pneumonia, meningitis and various gastrointestinal disorders

3. opportunistic infections occur outside of the intestine and require an alteration of the host by some mechanical, physiologic or infectious process before they can cause disease

4. true enteric pathogens – Salmonella, Shigella and Yersinia

5. potential danger – secondary bacteremia and endotoxic shock

Sites of infections with members of the enterobacteriaceae

Laboratory Diagnosis

Specimens – sputum, tissue, pus, body fluids, rectal swabs or feces

1. Culturea. cefsulodin-irgasan-novobiocin (CIN) – selective for

Yersiniab. isolation and differentiation of lactose fermenters

from nonlactose fermenters- Eosin Methylene Blue agar- MacConkey agar

c. Gram-negative broth (GN broth) – enrichment broth used to enhance isolation of enteric pathogens

d. Isolation and differentiation of Salmonella and SHigella; inhibition of normal flora coliforms- Hektoen enteric agar- Salmonella Shigella agar- Xylose-lysine-desoxycholate (XLD)

e. enrichment broth used to enhance recovery of Salmonella and Shigella- selenite broth- tetrathionate broth

2. Biochemical Testsa. Triple sugar iron Agar – to distinguish

morphologically similar bacteria of Enterobacteriacea all of which ferment glucose to an acid end product

b. IMVIC reactions – for the identification of lactose-fermenting members of Enterobacteriaceae. Escherichia, Enterobacter and Klebsiella.

i. Indol testii. Methyl red testiii. Voges Proskaeur testiv. Citrate utilization test

c. Urease test – for the identification of:

i. Rapid urease producers – Proteus and Morganella

ii. Weak urease producers – Klebsiella and some enterobacter

d. deaminase reactions – to identify organisms producing the enzyme deaminase such as Proteus, Providencia and Morganella

e. decarboxylase reactions – to determine the production of decarboxylases by bacteria

f. motility – determines the motility of bacteria through semisolid media

i. Nonmotile – Shigella and Klebsiellaii. Yersinia enterocolitica – nonmotile at

37oC but motile at 22oCg. ONPG reaction

i. Determines the presence of late or slow lactose fermenting strainso useful in detecting late-lactose

fermenting strains of Escherichia colio distinguishes Citrobacter species and

arizonae subspecies from similar Salmonella subspecies

o useful in the speciation of shigella

3. Serologic grouping of the O, H and K antigens to characterize certain enteric isolatesa. Salmonella and Shigella – for complete

identificationb. Other organisms – for epidemiologic purposes

Rapid, presumptive identification of gram negative bacteria

Lactose fermented rapidly

Escherichia coli- metallic sheen on differential media; motile, flat,

nonviscous colonies

Enterobacter aerogenes- raised colonies, no metallic sheen; often motile;

more viscous growth

Klebsiella pneumniae- very viscous, mucoid growth; nonmotile

Lactose fermented slowly

EdwardshiellaSerratiaCitrobacterArizonaProvidenciaErwiniaLactose not fermented

Shigella species- nonmotile; no gas from dextrose

Salmonella species- motile; acid and usually gas from dextrose

Proteus species- swarming on agar, urea rapidly hydrolyzed (smell

of ammonia)

Treatment1. factors in the difficult treatment of enterobacterial

infections underlying disease of patient emergence of resistant strains

2. The appropriate antibiotic must be chosen by: careful evaluation of the isolate’s

susceptibility pattern condition of the host site of infection

3. Treatment of shock – centers on the cardiovascular system

restoration of the intravascular volume digitalization administration of isoproterenol steroids, pressor amines and norepinephrine

SHIGELLA

1. Gram negative nonsporeformers2. none are motile, hence they do not contain

flagellar antigens3. some resemble anaerogenic type of coliform

bacilli and typhoid bacilli in that they ferment carbohydrates with the production of acid without gas

4. Differ from one another biochemically and immunologically

5. Facultative anaerobes; optimum temperature for growth is 37oC

6. Grow upon the ordinary nutrient (beef extract medium)

7. relatively resistant to the bacteriostatic action dyes

8. can grow in EMB, MacConkey, SS agar, deoxycholate agar

9. Only Shigella sonnei is a slow lactose fermenter

Properties that distinguish Shigella from most Salmonellae

1. lack of motility2. failure to produce gas during fermentation except

for some biotypes of Shigella flexneri 6 and rare strains of Shigella boydii 13

3. lack of lysine decarboxylase4. possess specific polysaccharide O antigens but no

H antigens5. much less invasion rarely causing bacteremia6. much narrower distribution in nature inhabiting

only the intestinal tracts of primates

Classification is based on fermentation of mannitolI. Nonfermenting group

– serogroup A – Shigella dysenteriaeII. Mannitol fermenters

– serogroup B – Shigella flexneri- serogroup C – Shigella boydii- serogroup D – Shigella sonnei

Group A – Shigella dysenteriae

1. Shigella dysenteriae type I – Shiga bacilluso produces an endotoxic LPS which is

responsible for O antigenicity of the bacillus

o unique in its production of an exotoxin, a neurotoxin, which affects the central nervous system

o Similar to diphtheria toxin in potency and mechanism of action

o Produces an enterotoxin which closely resembles cholera and coliform enterotoxins in its mechanism of action

2. Shigella dysenteriae type 2 – Schmitz bacilluso encountered in institutional and

other outbreaks of dysentery Group B – Shigella flexneri – Flexners bacillus, Hiss

and Russell’s bacillus1. Worldwide in distribution2. most commonly found of the dysentery bacilli

making up more than half of isolates Group C – Shigella boydii – Newcastle Manchester

bacillus- similar in pathogenicity to Shigella flexneri

but differs immunologically Group D – Shigella sonnei – Duval’s bacillus

1. most common species in the US – 72% of isolates

2. slow lactose fermenter, producing acid in one week to 10 days

Pathogenesis1. natural habitat of dysentery bacilli – large intestine

of humans – BACILLARY DYSENTERY2. infection limited to gastrointestinal tract3. blood stream invasion is rare4. highly communicable5. infective dose – less that 10 microorganisms

Cllinical Manifestations1. incubation period – 1 to 4 days2. begins with fever, abdominal cramping and pain,

and diarrhea3. Two stages of the infection

a. First stage – involves a watery diarrhea, which may last up to 3 days

b. Dysenteric phase – characterized by frequent stools, with red blood cells, white blood cells, and mucus presento the bacteria have invaded the

epithelial lining of the intestine, causing severe inflammation

DIFFERENTIAL DIAGNOSIS BETWEEN AMEBIC DYSENTERY AND TISSUE-INVASIVE BACTERIAL GASTRO-ENTERITIS

Amebic Dysentery

Tissue-invasive bacterial Colitis (Shegella dysentery)

Incubation period

Variable (usually 2-4 weeks or longer)

Short (24-72 hours)

Onset Insidious AcuteFever Usually absent,

unless complicated

Common, moderately high

Toxemia Usually absent Often presentDehydration Unusual usual

Hepatomegaly Common (if complicated or disseminated)

uncommon

Colonic mucosal changes

Gross findings: segmented: ulcers with undermined borders and normal intervening mucosa; lack of inflammation

Microscopic findings – trophozoites in flask-shaped ulcers; wet mount shows motile amoeba containing RBC

Diffuse erythema with loss of vascular pattern; mucopurulence, mild friability, occasional ophthoid ulcers

Edema, capillary congestion, focal hemorrhages, cyst, hyperplasia, goblet depletion, mononuclear polymorphonuclear leukocyte infiltrate; loss of epithelial cells with microulcerations

Stool appearance

Bloody mucoid semiformed

Bloody mucoid less formed

Fecal leucocytes (methylene blue staining)

Uncommon or few

Usually abundant with macrophages

Red cells Numerous and in clumps

Numerous, discrete

E. histolytica Hematophagous trophozoites present

absent

Indirect hemogglutination test (IHA)

Positive in invasive amebiasis (85-95% of cases)

negative

* Confusion in differentiating amebic cysts from fecal leucocytes and amebic cysts from fecal leucocytes and amebic trophozoites from large motile macrophages may occur with all the consequences of such a mistake

Epidemiology1. occurs in children from 6 months to 10 years of

age2. endemic disease in adults is frequently due to

contact with infected children3. epidemic outbreaks of disease are associated with

day care centers, nurseries and custodial institutions

4. transmitted by the fecal oral route, primarily by contaminated hands and less commonly in water or food – food, fingers, flies, feces

5. spreads rapidly in communities where sanitary standards and the level of personal hygiene are low

Laboratory DiagnosisSpecimens

1. fresh stool, mucus flecks, rectal swab – culture2. serum – serology

1. Culturea. EMB or MacConkey – colorless coloniesb. Salmonella-Shigella agar – colorless colonies

without black centersc. Hektoen enteric agar – green colonies without

black centers

2. Biochemical testsa. TSIA – acid butt, alkaline slant, no gas, no H2Sb. MR positivec. Citrate – negatived. ODC – negativee. ADH – negativef. Deaminase (phenylalanine) – negativeg. Urease – negativeh. Carbohydrate fermentation

o sucrose – negativeo salicin – negativeo adonitol – negativeo dulcitol – negativeo D-mannose – positive

3. Slide agglutination by specific Shigella antiseraReaction Dysenteri

aeFlexneri

Boydii

Sonnei

Bacilli penetrate epithelial cells of the colonic mucosa

Multiplication of bacilli

Microabscesses develop in the lamina propria

Formation of diphtheritic or pseudomembrane consisting of fibrin, WBC, debris, tissues and bacteria

ulcerations

Necrosis of mucosa due to action of enterotoxin

Granulation and scar formation

Fermentation of:LactoseMannitol

--

-+

-+

+-

ODC - - - +ONPG - - - +

Treatment1. supportive

a. prompt replacement of fluids and electrolytes

b. opiates should be avoided2. specific

a. potent specific antitoxin against Shigella dysenteriae

b. antimicrobialsi. Ampicillin or the analogue

amoxicillinii. Chloramphenicoliii. Tetracyclineiv. Norfloxacin or Ciprofloxacinv. Trimethoprim sulfamethoxazole

Prevention and Control1. mass chemoprophylaxis2. eliminate organism from reservoir hosts

a. sanitary control of water, food and milk; sewage disposal and fly control

b. isolation of patients and disinfection of excreta

c. detection of subclinical cases particularly in food handlers

SALMONELLAE

1. Gram negative, nonencapsulated, nonsporulating rods

2. All species are actively motile by means of peritrichous flagella except Salmonella pullorum and Salmonella gallinarum.

3. stain readily with the usual dyes such as methylene blue and carbol fuchsin

4. have simple nutritional requirements growing readily on the usual nutrient media

5. optimum temperature is 37 C but growth occurs at a reasonable rate at room temperature

6. facultative anaerobes growing equally well under

either aerobic or anaerobic conditions 7. characterized biochemically by failure to ferment

lactose or salicin and inability to liquefy gelatin or produce indole

8. ferments sugar with gas but there are anaerogenic strains of Salmonella enteritidis, Salmonella typhimurium and Salmonella paratyphi C

Kauffman White Classification scheme – based on agglutination tests with absorbed antisera, permitting identification of different O and H antigens in an unknown organism.

Salmonella Nomenclature

Salmonella subgroup

Former Genus

Subspecies

1 Salmonella Choleraesuis

2 Salmonella Salamae3a Arizona Arizonae3b Arizona Diarizonae4 Salmonella Houtenae5 Salmonella Bongor6 Salmonella idnica

Antigenic Structure1. somatic O antigen – LPS

a. associated with the cell substanceb. antibodies are mostly IgM

2. flagellar or H antigena. associated with the flagellab. antibodies are mostly IgG

3. Vi antigena. surface antigen of the K classb. occurs in Salmonella typhi, Salmonella

paratyphi A and B, and some strains of Citrobacter

c. blocks agglutination by antiserum against the O antigen

d. virulence antigen – presence associated with virulence and antibody; it is protective

4. Additional K antigen – R antigen

Host factors that contribute to the resistance to Salmonella infection

1. Gastric acidity2. normal intestinal microbial flora3. local intestinal immunity

Three Clinical Entities1. Salmonella gastroenteritis or Salmonella food

poisoning2. enteric fevers3. septicemia

Salmonella Gastroenteritis or Salmonella food poisoning

1. Most common manifestation of Salmonella infection

2. 8-48 hours after the ingestion ---------à nausea, headache, vomiting and profuse diarrhea with few leukocytes in the stools but rarely blood

3. Low grade fever4. Recovery within 2-3 days5. Blood stream invasion quite rare6. an infection not an intoxication because no toxin is

involved7. Large numbers of bacilli cause irritation of mucosa8. Causative agents

a. Salmonella typhimuriumb. Salmonella enteritidis(Gartners baciluus)c. Salmonella choleraesuis

Enteric Fevers1. Paratyphoid fever

a. relatively mild course with sudden chills but otherwise similar to a mild typhoid fever

b. most often milk borne and transmitted by carriers

c. predominant in younger age groupsd. causative agents

Salmonella paratyphi A – sewage contaminated food

Salmonella paratyphi B (Salmonella schottmulleri) – often in carriers

Salmonella paratyphi C (salmonella hirschfeldii) – tends to produce endocarditis

2. Typhoid fevera. Salmonella typhi – a strict parasite of

manb. Not only an intestinal infection but a

general invasion particularly the lymphatic system

Clinical Manifestations1. First week

a. gradual onset with increasing remittent feverb. dull, continuous headachec. anorexia, malaised. nonproductive coughe. epistaxisf. vague abdominal discomfortg. constipation

2. Second weeka. temperature sustained at 40oCb. appears acutely ill and weakc. dull, lethargic, expressionless faced. mental state normal to mental confusion to

deliriume. abdominal discomfort and distentionf. diarrhea more common with blood- Physical Findings

1) spleen enlarged and soft2) maculopapular lesions on anterior chest

and upper abdomen3) rose spots 2-3 mm. in diameter, blanch

on pressure lasting 2-4 days4) slow pulse rate or bradycardia inspite of

high temperature3. 3rd and 4th week – recovery when fever subsides;

death when untreated4. Complications

a. intestinal hemorrhageb. perforation and peritonitisc. splenic rupture

5. Typhoid carriersa. more in females than males between 50-59

yearsb. maybe convalescent carriers or chronic

carriersc. bacilli in the gall bladder or in the urinary

bladderd. best treated with massive ampicillin and

cholecystectomy

Septicemia1. may occur as a sequelae of enteric fever or rarely

from gastroenteritis or may have no intestinal focus

2. characterized by rapid rise with spiking temperature

3. Signs and symptoms refer to the area involved a. pneumoniab. meningitisc. conjunctivitisd. sinusitise. suppurative arthritisf. pyelonephritis

What makes Salmonellae pathogenic?1. rarely, formation of exotoxins. But all contain

endotoxins2. ECA or kunin antigen – inhibits phagocytosis and

decreases their susceptibility to bactericidal action of serum

3. fimbriae – mediate attachment of bacteria to intestinal mucosa, but have no pathogenic function extraintestinally

4. tendency to plasmid-mediated multiple drug resistance5. tendency to a carrier state especially in highly nutritive

environment of gall bladder6. Past infection and vaccination confer only partial or

temporary active immunity7. Enterotoxin similar to Escherichia is produced by

Salmonella typhimurium, causative agent of food poisoning.

EpidemiologySources of infection – food and drink contaminated with salmonellae

a. waterb. milk and other dairy products (ice cream, cheese,

custard)c. shellfishd. dried or frozen eggse. meats and meat productsf. recreational drugsg. animal dyesh. household pets

Laboratory DiagnosisSpecimens

Blood – culture + 1st week Bone marrow – culture Urine – culture + after 2nd week Stool – culture

o Typhoid fever + 2nd week or 3rd weeko Enterocolitis + 1st week

Serology

1. Culturea. enrichment cultures – selenite F broth or

tetrathionate brothb. differential media

o MacConkey – colorless colonies, nonlactose fermenter

c. selectiveo Salmonella Shigella agar – colorless

colonies, sometimes with black colonies due to H2S production (black center with clear periphery)

o Hektoen enteric agar – green with black centers (lactose negative; H2S positive)

o Bismuth sulfite agar – rapid detection of Salmonella typhi producing jet black colonies, black sheen or with dotted black or greenish gray

2. Biochemical identificationa. TSI: KI b. H2S: positivec. Motility: positived. MR: positivee. VP: negativef. Indole: negativeg. LDC: positiveh. Urease: negativei. Deaminase (phenylalanine): negativej. Fermentation of:

o sucrose – negativeo myoinositol - positiveo sucrose – positive

3. Serologya. slide agglutination with specific antisera –

clumpingb. Widal Test – tube dilution agglutination

o Patients serum + known antigeno Serum obtained twice 7-10 days aparto Significant reaction: rising titer or high

titer (1:160) 2nd-3rd week

Interpretation:o if predominantly O – active or acute

infection

o if predominantly H – past infection or vaccination

o If predominantly V – carrier state

c. Typhidot – the 60 minute testo Dot EIA for the rapid detection of

specific IgM and IgG antibodies

Result Clinical Interpretation1. IgM positive only Acute typhoid fever2. IgM and IgG positive

Acute typhoid fever (in the middle stage of infection)

3. IgG positive Previous infection (not due to typhoid); or relapse or reinfection

4. IgG and IgG Probably not typhoid

Treatment1. Enterocolits

a. fluid and electrolyte replacementb. antibiotic treatment not needed

o may prolong excretion of the organismso increase the frequency of carrier stateo select mutants resistant to the antibiotic

2. Enteric fevers and bacteremiasa. antimicrobial treatment

o ceftriaxone or ciproflaxin – treatment of choice

o chloramphenicolo trimethoprim-sulfamethoxazoleo ampicillin

b. carrierso ampicillin aloneo cholecystectomy combined with drug

treatment

Prevention and Control1. Through cooking of infected poultry, meats and

eggs2. carriers should not be allowed to work as food

handlers3. sanitary measures to prevent contamination of

food and water4. immunization

a. two injections of acetone killed bacterials suspensions of Salmonella typhi followed by booster injection

b. oral administration of a live avirulent mutant strain of Salmonella tyhi

-fin-

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