Tuesday, December 1, 2009

25 - EMB stain, EMB agar (Eosin Methylene Blue)

- Eosin methylene blue (EMB) is a selective stain for Gram-negative bacteria.

- It is a blend of two stains, eosin and methylene blue in the ratio of 6:1.

- A common application of this stain is in the preparation of EMB agar, a differential microbiological medium, which inhibits the growth of Gram-positive bacteria and provides a color indicator distinguishing between organisms that ferment lactose (e.g., E. coli) and those that do not (e.g., Salmonella, Shigella).

- Organisms that ferment lactose display "nucleated colonies" -- colonies with dark centers.

- Lactose fermentation produces acids, which lower the pH. This encourages dye absorption by the colonies, which are now coloured purple-black.

- Lactose non-fermenters may increase the pH by deamination of proteins. This ensures that the dye is not absorbed.

- On EMB if E.coli is grown it will give a distinctive metallic green sheen (due to the metachromatic properties of the dyes, E. coli movement using flagella, and strong acid end-products of fermentation). It is one of the few bacteria that reacts this way to EMB.

Sunday, November 1, 2009

24 - Lytic cycle and Lysogenic cycle

23 - Food poisoning - Diagnosis

  • Short incubation (ie, within 1 d, usually less than16 h)
    • Chemical causes (ultrashort incubation): The onset of nausea, vomiting, and cramps within 1-2 hours is observed in poisonings involving metal, fish-associated toxins (eg, scombroid, ciguatera), shellfish-associated toxins, monosodium glutamate, or mushrooms. The toxic agent in shellfish-related and ciguatera-related disease is derived from dinoflagellate organisms present in the fish or shellfish. Note that neurologic symptoms can present weeks later. Amanita mushrooms can lead to hepatorenal failure.
    • Bacterial causes - Emetic syndrome (1-6 h)
      • S aureus: Nausea and vomiting are caused by the action of preformed enterotoxins A-E.
      • Bacillus cereus (emetic syndrome, indistinguishable from staphylococcal food poisoning): This spore-forming rod is present in raw rice grains. The emetic toxin is a preformed heat-stable toxin produced upon germination of the spores.

    • Bacterial causes - Diarrheal syndrome (8-16 h)
      • B cereus (diarrheal syndrome): The diarrheal toxin is a heat-labile toxin formed after sporulation.
      • Clostridium perfringens type A: Diarrhea and abdominal cramps occur within 1 day of ingestion of cooked meat stored at 15-60°C. Slow cooling allows heat-activated spores to germinate and to elaborate the enterotoxin.

  • Intermediate incubation (1-3 d)
    • Diarrheal disease: This category comprises bacterial and viral infectious pathogens. The clinical presentation depends on the target organ (ie, small bowel or large bowel), which varies depending on the pathogen.
    • Diarrheal disease, large bowel enteritis: Fever and constitutional symptoms usually accompany the diarrhea caused by invasive pathogens in the large bowel. Dysentery, bloody stools with mucous, and cramps or tenesmus are typical.
      • Campylobacter jejuni: This is a leading cause of bacterial food-borne illness in the United States. Vomiting is uncommon, and the illness is short and self-limiting.
      • Shigella species: Shigellae cause the prototypical diarrheal syndrome with blood, mucous, and pain that is termed bacillary dysentery. Tenesmus and small-volume stools are typical. Toxemia may be severe, occasionally causing seizures in children.
      • Enteroinvasive E coli (EIEC): Several serotypes of diarrheagenic E coli possess Shigella -like invasiveness factors that allow mucosal invasion. The disease is a febrile dysentery that mimics shigellosis.
      • Salmonella species, nontyphoidal salmonellosis: This is a zoonotic infection acquired from bovine or poultry reservoirs and is very common in the United States. The illness can range from mild nonbloody diarrhea to a severe dysenteric illness.
      • Salmonella species, enteric (typhoid) fever: In the United States, enteric fever occurs in travelers or recent immigrants and is a systemic toxic illness. Salmonella typhi has an exclusively human reservoir and is acquired either via ingestion of a large inoculum in food or contaminated water or from personal contact with a carrier.
      • Vibrio parahaemolyticus: Although it is a common worldwide pathogen, in the United States, V parahaemolyticus infection is restricted geographically to the Atlantic and Gulf coasts. The diarrhea is profuse and watery, and blood is not commonly present in the stool.

    • Diarrheal disease, small bowel enteritis
      • Enterotoxigenic E coli (ETEC): Enterotoxin-producing strains of E coli are the most common cause of traveler's diarrhea. The diagnosis is clinical; fever and bloody stools are typically absent.
      • Vibrio cholerae (01 and non-01 strains): Cholera is likely only in endemic areas and during epidemics. The profuse diarrhea and vomiting can lead to dehydration and prostration.
      • Viral agents (Norwalklike viruses, rotavirus, adenoviruses, astroviruses, caliciviruses): Vomiting and headache accompany the diarrhea and fever more commonly with viral than with bacterial infections.
      • All the large bowel pathogens also secrete enterotoxins that induce profuse watery small bowel diarrhea in some patients.

    • Botulism: Nausea, vomiting, skeletal muscle paralysis, and autonomic symptoms occur within 18-36 hours of ingestion of food containing Clostridium botulinum. The disease is mediated by preformed toxin in older children and adults, but it may follow ingestion of spores in infants. Diarrhea occurs only in approximately 5% of patients; instead, constipation may be noted. Infants with botulism present with muscular weakness that manifests as weak cry, difficulty sucking and swallowing, or respiratory failure. Upon examination, the baby has profound hypotonia but may be alert.

  • Long incubation (3-5 d)
    • Enterohemorrhagic E coli (EHEC): These strains of E coli cause hemorrhagic colitis with a 15% risk of progression to hemolytic-uremic syndrome (HUS) in children, which is the result of cytotoxins termed verotoxins or Shigalike toxins. E coli O157:H7 is one of many such cytotoxin-producing E coli strains that reside in the gut of cattle. Although these organisms can cause mild nonbloody diarrhea, hemorrhagic colitis is the usual symptom. Mild abdominal pain, malaise, and transient fever are followed by watery diarrhea. Bloody stools and more severe abdominal pain ensue several days later. Paucity of fever is a diagnostic clue. When HUS occurs, its onset is 5-13 days after the onset of diarrhea.
    • Yersinia species
      • Yersinia enterocolitica most often causes a febrile illness with abdominal pain due to mesenteric lymphadenitis in which diarrhea is not prominent. The illness can mimic appendicitis. The illness may be prolonged, lasting 2-3 weeks. In infants, a diarrheal illness is common, with occasional septicemia. The diagnosis is made with blood and stool cultures. Treatment is indicated only for infants with septicemia.
      • The symptom complex for Yersinia pseudotuberculosis infection includes fever, rash (scarlatiniform or erythema nodosum), and abdominal pain.

  • Very long incubation (1-4 wk): This category comprises parasitic food-borne diseases (FBDs), but shorter incubation periods can occur, especially in Entamoeba histolytica infection.
    • Parasitic
      • Giardiasis: The spectrum of illness ranges from asymptomatic carriage to acute watery diarrhea, but a subacute intermittent diarrheal illness is also common.
      • Amebiasis: E histolytica is a protozoan that causes dysentery and extraintestinal, most commonly hepatic, abscesses.
      • Cryptosporidiosis: The organism Cryptosporidium parvum causes a diarrheal illness with fever and abdominal pain.
      • Cyclosporiasis: Frequent watery stools, which can be accompanied by fever and a relapsing course, characterize this FBD caused by Cyclospora cayetanensis.
      • Trichinosis: This is a rare illness, caused by Trichinella spiralis, that is acquired by ingestion of contaminated or raw pork, bear, or moose meat. GI tract symptoms are followed by muscle inflammation and periorbital edema.
      • Cysticercosis: This infection is caused by the larval stage of pork tapeworm and is most often acquired by ingestion of food or water contaminated with the ova of the tapeworm rather than from eating raw pork.
      • Anisakiasis, fish tapeworm, and flatworm infections: These uncommon worm infestations occur after consumption of certain types of raw fish.

    • Bacterial
      • Listeriosis: Diarrhea in Listeria monocytogenes infection may be mild, but systemic symptoms are prominent. The diarrhea has a short incubation period (<48 h), but symptoms of systemic spread could appear weeks later. The major risk is that of maternal infection during pregnancy. Neonatal sepsis and meningitis follow amniotic fluid infection. Older children and adults can develop meningitis. The infection is a particular hazard to individuals who are immunocompromised.
      • Brucellosis: This is a febrile illness now only rarely acquired in the United States. The food source is raw or unpasteurized milk or cheese, most commonly from goats (Brucella melitensis).

    • Viral: The incubation period of hepatitis A is 15-50 days for this viral hepatitis transmitted via the fecal-oral route.
    • Protozoal, toxoplasmosis: A febrile and subacute lymphadenitis results from ingestion of undercooked meat. A nonspecific illness with systemic symptoms and generalized lymphadenopathy can occur in healthy individuals, or an asymptomatic infection can result. Persons who are immunocompromised can develop CNS infection.

Thursday, October 22, 2009

22 - Runyon classification (of Nontuberculous Mycobacteria)

The Runyon classification of nontuberculous mycobacteria based on the rate of growth, production of yellow pigment and whether this pigment was produced in the dark or only after exposure to light.

It was introduced by Ernest Runyon in 1959.

On these bases, the nontuberculous mycobacteria are divided into four groups:


The first three groups are classified as "Slowly growing Mycobacteria".

Runyon I: Photochromogens
Runyon I organisms (photochromogens) are slow growing, and produce a yellow-orange pigment when exposed to light. Mycobacterium kansasii, Mycobacterium marinum, Mycobacterium simiae, Mycobacterium asiaticum.

Runyon II: Scotochromogens
Runyon II organisms (scotochromogens) are slow growing, and produce a yellow-orange pigment in light or in the dark. Mycobacterium scrofulaceum, Mycobacterium gordonae, Mycobacterium szulgai, Mycobacterium xenopi, Mycobacterium celatum, Mycobacterium flavescens.

Runyon III: Nonchromogenic
Runyon III organisms are slow growing, and do not produce pigment. Mycobacterium avium-intracellulare, Mycobacterium xenopi, Mycobacterium terrae, Mycobacterium paratuberculosis, Mycobacterium shimoidae, Mycobacterium genavense


Runyon IV organisms are rapid growing for mycobacteria (colonies in 5 days). They do not produce pigment. Mycobacterium fortuitum, Mycobacterium peregrinum, Mycobacterium abscessus, Mycobacterium chelonae,   Mycobacterium thermoresistibile,  Mycobacterium peregrinum .

Some rapidly growing mycobacteria are considered "late-pigmenting".

21 - MacConkey agar


MacConkey (also McConkey) agar is a (differential) culture medium designed to grow Gram-negative bacteria and stain them for lactose fermentation.

It contains bile salts (to inhibit most Gram-positive bacteria, except Enterococcus and some species of Staphylococcus), crystal violet dye (which also inhibits certain Gram-positive bacteria), neutral red dye (which stains microbes fermenting lactose), lactose and peptone.

Alfred Theodore MacConkey developed it while working as a bacteriologist for the Royal Commission on Sewage Disposal.

Acting as a visual pH indicator, the agar distinguishes those Gram-negative bacteria that can ferment the sugar lactose (Lac+) from those that cannot (Lac-).

This medium is also known as an "indicator medium" and a "low selective medium". Absence of electrolytes serves to inhibit swarming by Proteus species.


By utilizing the lactose available in the medium, Lac+ bacteria such as Escherichia coli, Enterobacter and Klebsiella will produce acid, which lowers the pH of the agar below 6.8 and results in the appearance of red/pink colonies.


Non-Lactose fermenting bacteria such as Salmonella, Proteus species and Shigella cannot utilize lactose, and will use peptone instead. This forms ammonia, which raises the pH of the agar, and leads to the formation of white/colorless colonies.


Some organisms ferment lactose slowly or weakly, and are sometimes put in their own category. These include Serratia and Citrobacter.

A variant, Sorbitol-MacConkey agar, (with the addition of additional selective agents) can assist in the isolation and differentiation of enteropathogenic E. coli serotypes such as E. coli O157:H7.

20 - Nagler's Reaction

At least 12 different toxins have been identified in strains of Cl. perfringens. One of these is the alpha-toxin, a lecithinase that hydrolyses the phospholipid lecithin (a component of cell membranes) to a diglyceride and phosphorylcholine.

The activity of the alpha-toxin can be demonstrated by growth on agar containing egg yolk (as a source of lecithin): an opaque zone representing insoluble diglyceride, becomes evident around colonies of Clostridium perfringens (the Nagler reaction). The activity of alpha-toxin is inhibited by anti-alpha-toxin antibody (generated by vaccination with alpha-toxoid).

The photograph provided above shows a nutrient agar plate enriched with egg yolk. Anti a-toxin was spread over half of the plate (left half) and a heavy inocula of Cl. perfringens and Cl. sporogenes were streaked across the plate at right angles to the anti alpha-toxin boundary.

Note the opaque zone on the upper right half, around the colonies of clostridium perfringens, due to insoluble diglyceride formed as a result of action of lecithinase (alpha toxin) on the lecithin of egg. THIS IS THE NAGLER'S REACTION.

The upper left half does not show this phenomenon because of presence of anti alpha toxin .antibodies.

19 - Schuffner's dots

Fine, round, uniform red or red-yellow dot's (as coloured with Romanovsky stains) characteristically observed in erythrocytes infected with Plasmodium vivax and P. Ovale, but not ordinarily found in P. Malariae and P. Falciparum infections.

Tuesday, October 6, 2009

18 - Bacteria That Cause Disease in Humans - Major categories and groups

I. Gram-negative eubacteria that have cell walls  
   Group 1: The spirochetes Treponema
  Group 2: Aerobic/microaerophilic, motile helical/vibroid gram-negative bacteria Campylobacter
  Group 3: Nonmotile (or rarely motile) curved bacteria None
  Group 4: Gram-negative aerobic/microaerophilic rods and cocci Alcaligenes
Bacteroides (some species)
  Group 5: Facultatively anaerobic gram-negative rods Escherichia (and related coliform bacteria)
  Group 6: Gram-negative, anaerobic, straight, curved, and helical rods Bacteroides
  Group 7: Dissimilatory sulfate- or sulfur-reducing bacteria None
  Group 8: Anaerobic gram-negative cocci None
  Group 9: The rickettsiae and chlamydiae Rickettsia
  Group 10: Anoxygenic phototrophic bacteria None
  Group 11: Oxygenic phototrophic bacteria None
  Group 12: Aerobic chemolithotrophic bacteria and assorted organisms None
  Group 13: Budding or appendaged bacteria None
  Group 14: Sheathed bacteria None
  Group 15: Nonphotosynthetic, nonfruiting gliding bacteria Capnocytophaga
  Group 16: Fruiting gliding bacteria: the myxobacteria None
II. Gram-positive bacteria that have cell walls  
  Group 17: Gram-positive cocci Enterococcus
  Group 18: Endospore-forming gram-positive rods and cocci Bacillus
  Group 19: Regular, nonsporing gram-positive rods Erysepelothrix
  Group 20: Irregular, nonsporing gram-positive rods Actinomyces
  Group 21: The mycobacteria Mycobacterium
  Groups 22–29: Actinomycetes Nocardia
III. Cell wall-less eubacteria: The mycoplasmas or mollicutes  
   Group 30: Mycoplasmas Mycoplasma
IV. Archaebacteria  
  Group 31: The methanogens None
  Group 32: Archaeal sulfate reducers None
  Group 33: Extremely halophilic archaebacteria None
  Group 34: Cell wall-less archaebacteria None
  Group 35: Extremely thermophilic and hyperthermophilic sulfur metabolizers None

Friday, July 24, 2009

17 - Germ Tube test

The yeast is incubated in horse serum at 37oC for 2 hours and the cells examined microscopically. Candida albicans, and the rarer oral pathogen C. dubliniensis, are the only species of the genus Candida that produce a short germ tube under these conditions as seen in the figure. C. albicans represents 80% of all clinical isolates. Other species of the genus Candida are germ tube-negative and require further tests to identify them.

Sunday, April 19, 2009

16 - Pathogenic Virus Families affecting Humans

Virus Families Pathogenic for Humans
Representative Viruses
Type of RNA/DNA
Lipid Envelope
RNA Viruses
Hepatitis A virus
(+) RNA
Norwalk agent
Hepatitis E virus
(+) RNA
Rubella virus
Eastern equine encephalitis virus
Western equine encephalitis virus
(+) RNA
Yellow fever virus
Dengue virus
St. Louis encephalitis virus
West Nile virus
Hepatitis C virus
Hepatitis G virus
(+) RNA
(+) RNA
Rabies virus
Vesicular stomatitis virus
(–) RNA
Marburg virus
Ebola virus
(–) RNA
Parainfluenza virus
Respiratory syncytial virus
Newcastle disease virus
Mumps virus
Rubeola (measles) virus
(–) RNA
Influenza A, B, and C viruses
(–) RNA, 8 segments
California encephalitis virus
Sandfly fever virus
(–) RNA, 3 circular segments
Lymphocytic choriomeningitis virus
Lassa fever virus
South American hemorrhagic fever virus
(–) RNA, 2 circular segments
Colorado tick fever virus
ds RNA, 10–12 segments
Human T-lymphotropic virus types I and II
Human immunodeficiency virus types 1 and 2
(+) RNA, 2 identical segments
DNA Viruses
Hepatitis B virus
ds DNA with ss portions
Parvovirus B19
ss DNA
Human papillomaviruses
JC virus
BK virus
ds DNA
Human adenoviruses
ds DNA
Herpes simplex virus types 1 and 2b
Varicella-zoster virusc
Epstein-Barr virusd
Human herpesvirus 6
Human herpesvirus 7
Kaposi's sarcoma–associated herpesvirusf
ds DNA
Variola (smallpox) virus
Orf virus
Molluscum contagiosum virus
ds DNA
aIncluding the coronavirus causing severe acute respiratory syndrome (SARS).
bAlso called human herpesvirus (HHV) 1 and 2, respectively.
cAlso called HHV-3.
dAlso called HHV-4.
eAlso called HHV-5.
fAlso called HHV-8.
Abbreviations: ds, double-strand; ss, single-strand.

Sunday, April 5, 2009

15 - Facultative anaerobes part 5 ( PASTEURELLACEAE )


• Pasteurella

o Pathogen of domestic animals (septacemia = cattle; fowl cholera = chickens)

o P. multocida transmitted to humans by dog / cat bites.

o Nutritionally fastidious; requires complex media for growth in the laboratory

• Haemophilus:

o Common inhabitants of upper respiratory tract mucous membrane, mouth, vagina, GI.

o Requires X factor (unknown component of hemoglobin) for synthesising parts of the cytochrome system; eg choclate agar (blood heated & gives brown color) supllemented with NAD or NADP (V factor).

o H. influenzae - flu like

o H. ducreyi -- veneral disease aka soft chancre

o H. suis-- hog influenza

14 - Facultative anaerobes part 4 ( VIBRIONACEAE )


• Curved to comma shaped

• Polarly flagellated

• Facultative anaerobes

o Aerobic & therefore catalase & oxidase +ve (similar to aerobic Pseudomonads)
o Fermenters ie anaerobic

• Widespread distribution (fresh water, sea water) & human & animal intestinal tracts); not indicators of fecal pollution.

• 3 genera (Vibrio, Aeromonas, Photobacterium)

o Vibrio Cholera (aka Asiatic cholera)
 Two biotypes of serogroup O:1 known (classic cholera & Eltor [Mecca camp]); Eltor endemic in coastal regions eg India, Bangladesh & South America; USA non-O1 serogroup due to consumption of contaminated seafood.
 Transmitted thro' sewage contaminated water & food; rice water stools
 Multiplies in intestinal epithelial cells (but not invasive to other tissues) & produces an enterotoxin (binds irreversibly to epithelial cells stimulating the production of cyclic which changes cell permeability leading to the secretion of water & electrolytes into the lumen. 12 to 20 litres of water lost-- dehydration; replacement of electrolytes & water necessary is the best treatment; Tetracycline decreases disease period; vaccines provide short immunity but natural infections provide a longer duration (against enterotoxin & cells)
 Culture in alkaline medium (high pH decreases the growth of other bacteria)
 Vibrio parahemolyticus
 Habitat is salt water and requires > 2% NaCl for growth in laboratory media (halophilic) Gastroenteritis from contaminated shell fish, crabs; USA, Japan, Hawaii Large nos. within 24 hr & recovery follows in a few days.

o Aeromonas
 Mainly pathogens of fresh water fish, turtles & frogs.
 May be encountered as causative agents of human diseases.

o Photobacterium
 Interesting due to the property of bioluminescence (aka luminescent bacteria); ATP + electron transport chain + enzyme luciferase = luminescence
 Normal flora of some fish (flash light fish); behavioral activities (mating, protection)

13 - Facultative anaerobes part 3 ( ENTEROBACTERIACEAE )

• Escherichia :

o Most common inhabitants of intestinal tracts.

o Food & water presence indicates fecal contamination; indicator bacterium

o Endogenous spread (septacemia, urinary tract infections, meningitis) or food-water borne transmission (gastroenteritis, travellers diarrhea)

o Antigenic composition complex: 170, 57 H and several K antigens (epidemiological & specific serotypes associated with increased virulence (H157:O7)

o Septicimia: Focus of infection is urinary tract or intestine and spreads to bloodstream (higher incidence of death in immunocmpromised patients)

o Urinary tract infection: 80% = community aquired or hospital aquired. 10 specific serotypes usually involved (eg 01, 02, 016, 075) & origin is from gastrointestinal tract.

o Pathogenecity --serotypes more adhesive to uroepithelial cells than other serotypes

o Meningitis: E.coli & B streptococci cause 75%. Serotype K1

o Gastroenteritis: 4 groups; enterotoxigenic (ETEC) causes travellers & infant diarrhea, pathogenecity similar to V. cholera toxin ie stimulates hypersecretion of fluids & electrolytes in gastrointestinal tract , not serotype specific, produce heat labile & heat stable plasmid mediated enterotoxins, not serotype specific (NA probing, tissue culture or animal models & not serotyping for identification. Enteroinvasiveness (EIEC)

• Klebsiella : 

o Capsulated

o K. pneumoniae-- pneumonia (old or immunocompromised eg alcoholics); also causes septecimia in hospital pediatric wards

• Serratia : 

o S. marcesens- red pigmented colonies

o Causes hospital aquired infections (nosocomial) like Enerobacter

o Urinary & respiratory tract infection

o Isolated from sterile saline solution, catheters


• Proteus : 

o Extremely motile

o P. mirabilis-- urinary tract infection; produces urease & splits urea to CO2 and NH3 to increase pH & therefore facilitate renal stone formation & uroepithelium toxicity.

o Pili decreases phagocytosis & therefore decreases virulence unlike E. coli .

• Erwinia : 

o Plant pathogens (phytopathogens)

o Produce pectinase which hydrolyse pectin between plant cells & cause plants to rot

12 - Facultative anaerobes part 2 ( ENTEROBACTERIACEAE )

• Salmonella :

o 2000 serotypes based on H (flagella protein), O (outer membrane protein) & K (Capsular protein)

o Antibodies obtained commercially for serotyping using Kauffmann-White scheme.

o Some Salmonella serotypes are host specific (humans= S. typhi S. paratyphi; animals (S. cholerae-suis but cause severe disease if transmitted to humans) & some others not host specific (affect humans & animals)

o Food borne disease mainly thro' poultry, eggs & dairy products

o Exposure to Salmonella is frequent but requires a high dose of 1 to 100 million (eg improperly refrigerated foods; low dose for immunocompromised, very young and the elderly or with decreased gastric acidity

o Disease is due to microbial growth in body tisssue & not by ingestion of foods contaminated with toxins as a result of microbial growth

o Salmonella infections occurs in one of four forms:

 Gastoeneteritis (S. enteritis) most common form; eating contaminated foods; elevated temperatures, cramps & head ache. Symptoms disappear spontaneously in 2 to 7 days
 Septicimia--All Salmonella can cause bacterimia (S. cholera-suis, S. typhi & S. paratyphi)
 Enetric fever (S. typhi- typhoid fever & S. paratyphi- paratyphoid fever)-- Similar symptoms but paratyphoid fever milder; Transmission by consumption of contaminated foods --> multiplication in intestine (endothelial cells)--> invades lymphatic tissues to blood stream & kidneys (excreted in urine)
 Asymptomatic carriage-- Establishes in gall bladder (ressists bile & bile salts); continuous feedback into intestine; known as carrier state (2-5% typhoid patients become carriers & excrete 1 to 1000 million S. typhi /gm feces & also in urine); Carrier state is important in transmission of the disease. Mary Mallon (1901), aka Typhoid Mary, was the first carrier case to be detected; In 15 years infected 200 & jailed to prevent spred of typhoid fever. Disease has declined due to curative & preventative measures (heat killed injectable vaccines or the new live oral vacciine; same efficacy but later has less side effects

• Shigella : 

o Simple taxonomy (4 species & 32 serotypes) unlike Salmonella

o Found in humans, apes & monkey only unlike Salmonella

o Cause bacillary dysentry (shigellosis); S. sonnei (industrialized nations) & S. flexneri (developing nations); S. sonnei mild but S. dysenteriae (aka Shiga bacillus produces Shiga toxin) severe.

o Shigella enter endothelial cells (intestine) & multiply causing tissue damage; exotoxins stop host cell protein synthesis. Rarely (with the exception of S. dysenteriae) cause septicimia. 20 bowel motions a day.

o Transmission by fecal-oral route thro' contaminated hands & usually not by food-water

o Low dose required-- 200 Shigella cells can establish disease

11 - Facultative anaerobes part 1 ( ENTEROBACTERIACEAE )

• Produce acid & gas anaerobically
• Medically very important
• 3 families:

General comments - 
• Most important heterogenous collection of medically significant bacteria (27genera & 110 sp)
• Ubiquitous- soil, water, vegetation (saprophytes) & some normal flora of humans & animals but opportunistic (E.coli, Klebsiella pneumoniae, Proteus mirabilis) and some always pathogenic (Salmonella, Shigella & Yersinia pestis).
• Infections can originate from an animal reservoir (Salmonella), from a human carrier (S. typhi, Shigella) or by endogenous spread of the organism in a suspetible patient (E. coli)
• Readily culturable on non-selective media from sterile specimens (spinal fluid, tissues), selective media eosin methylene blue (EMB), MacConkey agar for contaminated specimens (sputum, feces), highly selective media (eg stool samples-- normal flora mask) & cold enrichment technique (stool sample + saline --> 4 C / 2 weeks; most enerobacters killed but not Yersinia enterocolitica (slow grower)
• Identification based on fermentation products (types & ratio) eg Butanediol (Enterobacter); mixed acids (E. coli; Salmonella & Shigella-- ratios of products important) , serological properties but cross-reactions limit usefulness [flagella (H), lipopolysaccaharide (O) & capsular (K); eg E.coli (170H, 56H & numerous K), biochemical propeties identification within 4 to 24 hrs; Capsules-- Klebsiella
• Treatment with antibiotics (E. coli, Proteus miabilis) but some highly ressistant & in some cases not recommended (S. enteriditis, prolongs carrier state). Antibiotic ressistant bacterial infections commonly hospital acquired (not community aquired)
• Pathogenecity : 
• Endotoxins (lipid A of LPS) released from cell lysis , flagella, pilli & fimbrae in adherence & attachment; capsules --weakly antigenic & protect against antibody mediated phagocytosis

Monday, March 16, 2009

10 - AIIMS june 1999 microbiology mcqs

1q: which of the following is not seen in HIV patient with CD4 count less than 100 per microlitre , who has not productive cough ?

a. mycobacterium tuberculosis
b. pneumocystis carinii
c. mycoplasma pneumoniae
d. cryptococcal infection

2q: risk of pneumococcal meningitis is seen in ?

a. post splenectomy patient
b. patient undergone neurosurgical intervention
c. patient following cardiac surgery
d. patient with hypoplasia of lung

3q: which fungal infection is commonest to occur in neutropenia ?

a. candida
b. histoplasma
c. aspergillus niger
d. aspergillus fumigatus

4q: all are true about rickettsia except ?

a. obligate intracellular
b. gram positive bacillus
c. arthropods are vectors
d. weil felix test is diagnostic

5q: all are true about enterohaemorrhagic E.coli except ?

a. sereny test positive
b. may cause diarrhea
c. can cause hemolytic uremic syndrome
d. verocytotoxin is produced

6q: all of the following are true regarding typhoid except ?

a. urinary carriers are more dangerous
b. Vi ab is used for detecting carrier
c. Vi is seen in normal population
d. Urine carrier is associated with anomalies

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