GRAM NEGATIVE BACILLI AND COCCOBACILLI (ENTEROBACTERIACEAE) Flashcards
What are the key metabolic characteristics of Enterobacteriaceae?
Ferment glucose
Reduce nitrate to nitrites
Oxidase-negative
types of antigens found in Enterobacteriaceae
Found in the bacterial cell wall
Somatic or O antigen
types of antigens found in Enterobacteriaceae
Found in the flagella
Flagellar or H antigen
types of antigens found in Enterobacteriaceae
Found in the bacterial capsule
Capsular or K antigen
What are the differentiating and selecting agents in MacConkey (MAC) agar?
Differentiating Agent: Lactose
Selecting Agents: Crystal violet, Bile salts
How do lactose-positive and lactose-negative colonies appear on MacConkey (MAC) agar?
Lactose Positive: Dark pink
Lactose Negative: Transparent
What are the differentiating and selecting agents in Eosin Methylene Blue (EMB) agar?
Differentiating Agents: Sucrose, Lactose
Selecting Agents: Eosin Y, Methylene blue
How do lactose-positive and lactose-negative colonies appear on Eosin Methylene Blue (EMB) agar?
Lactose Positive: Green sheen, purplish, and brownish amber
Lactose Negative: Transparent
What are the differentiating and selecting agents in Hektoen Enteric (HE) agar?
Differentiating Agents: Lactose, Sucrose, Salicin
Selecting Agent: Bile salts
How do lactose-positive and lactose-negative colonies appear on Hektoen Enteric (HE) agar?
Lactose Positive: Salmon
Lactose Negative: Green to blue, H2S positive colonies have black centers
What are the differentiating and selecting agents in Salmonella-Shigella (SS) agar?
Differentiating Agent: Lactose
Selecting Agents: Brilliant green, Bile salts
How do lactose-positive and lactose-negative colonies appear on Salmonella-Shigella (SS) agar?
Lactose Positive: Red
Lactose Negative: Transparent, H2S positive colonies have black centers
What are the differentiating and selecting agents in Xylose Lysine Deoxycholate (XLD) agar?
Differentiating Agents: Lactose, Sucrose, Xylose
Selecting Agent: Bile salts
How do lactose-positive and lactose-negative colonies appear on Xylose Lysine Deoxycholate (XLD) agar?
Lactose Positive: Yellow
Lactose Negative: Transparent on red medium, H2S positive colonies have black centers
Ability to ferment glucose, lactose, and sucrose
Production of hydrogen sulfide
Triple Sugar Iron Agar (TSI)
What indicators are used in the Triple Sugar Iron Agar (TSI) test?
Phenol red for acidification
Ferrous sulfate for H2S formation
It tests the ability of the organism to produce the enzyme tryptophanase and deaminate tryptophan to indole, pyruvic acid, and ammonia.
Indole Production
How are positive and negative results indicated in the Indole Production test?
Positive: Pink to wine-colored ring after addition of reagents
Negative: Yellow color
It detects bacteria that metabolize glucose by the mixed acid pathway, leading to a decrease in the pH of the medium.
Methyl Red
How are positive and negative results indicated in the Methyl Red test?
Positive: Red color (when medium reaches a pH of 4.5)
Negative: Yellow color
It detects end products of glucose fermentation by the butylene glycol pathway.
Voges-Proskauer test
How are positive and negative results indicated in the Voges-Proskauer test?
Positive: Red color (presence of acetoin)
Negative: Yellow color
It determines if an organism can utilize sodium citrate as a sole source of carbon.
Citrate Utilization Test
What indicator is used in the Citrate Utilization Test?
Bromthymol blue.
How are positive and negative results indicated in the Citrate Utilization Test?
Positive: Growth on the medium with or without color change (green to blue)
Negative: Absence of growth
It determines the ability of the organism to reduce nitrate to nitrite.
Nitrate Reduction
How is a positive result indicated in the Nitrate Reduction test?
Red color.
It uses a semisolid medium to demonstrate motility.
Motility test
How is a positive result indicated in the Motility test?
Haziness along the stab line.
It detects organisms that produce urease, which hydrolyzes urea to form ammonia.
Rapid Urease Test
How is a positive result indicated in the Rapid Urease Test?
Bright pink color.
It detects slow lactose fermenters.
ONPG (beta-galactosidase)
How are positive and negative results indicated in the ONPG test?
Positive: Yellow color within 20 minutes to 24 hours
Negative: Colorless after 24 hours
It determines the ability of an organism to oxidatively deaminate phenylalanine to phenylpyruvic acid.
Phenylalanine Deamination
How are positive and negative results indicated in the Phenylalanine Deamination test?
Positive: Intense green color
Negative: No color change
What indicator is used in Carbohydrate Fermentation tests?
Phenol red.
How is a positive result indicated in Carbohydrate Fermentation tests?
Yellow color.
What are the key biochemical characteristics of Escherichia coli?
Indole and lactose positive
IMViC: + + - -
How does Escherichia coli appear on EMB or MacConkey agar?
greenish metallic sheen.
What is the most common cause of urinary tract infections (UTIs) in females?
Escherichia coli.
What are the key factors in the pathogenesis of Escherichia coli?
Pili
Flagellum (H antigen)
Capsule (K antigen)
Endotoxin (O antigen)
Enterotoxin
What condition is caused by Enterotoxigenic E. coli (ETEC)
Watery diarrhea.
What condition is caused by Enteropathogenic E. coli (EPEC)?
Watery diarrhea of long duration, mostly in infants, often in developing countries.
What conditions are caused by Enterohemorrhagic E. coli (EHEC)?
Bloody diarrhea, hemorrhagic colitis, and hemolytic uremic syndrome (HUS).
What condition is caused by Enteroinvasive E. coli (EIEC)?
Bloody diarrhea.
What condition is caused by Enteroadherent E. coli (EAEC)?
Persistent watery diarrhea in children and patients with HIV.
IDENTIFY
Lactose negative
Non-lactose fermenting
Non-motile
Anaerogenic
Have O antigen
Cultured in XLD
Shigella
How is Shigella transmitted?
Via the fecal-oral route.
What disease is caused by Shigella and what are its symptoms?
Bacillary dysentery
-Penetrate the epithelial cells in the small intestine
-Incubation period: 1-4 days
-Fever abdominal cramps, followed by watery then bloody diarrhea
-Diarrhea frequently resolves in 2 or 3 days
Which Shigella species is the most severe?
Shigella dysenteriae (Group A).
Which Shigella species is the most common?
Shigella sonnei (Group D).
How many Shigella organisms are needed to cause disease?
Fewer than 200 organisms.
IDENTIFY
Facultative gram-negative rods with a large polysaccharide capsule
Non-motile
Urease positive
Ornithine negative
VP positive
Klebsiella
Where are Klebsiella commonly found in the human body?
Inhabitants of the nasopharynx and gastrointestinal tract (GIT).
What type of pneumonia is caused by Klebsiella and what are its symptoms?
Type: Necrotizing pneumonia
Symptoms: Thick, bloody sputum (“currant jelly” sputum), usually nosocomial, most common cause in alcoholics
What other infection is commonly caused by Klebsiella?
Urinary tract infection (UTI).
How many Salmonella organisms are typically needed to cause an infection?
More than 100,000 organisms
IDENTIFY
Facultative gram-negative rods
Non-lactose fermenting
Cultured on XLD medium
H2S positive
Lysine positive
Indole negative
Urea negative
SALMONELLA
What is the Widal test used for?
It detects antibodies in the patient’s serum.
Salmonella typhi typically found?
In the human colon only.
Salmonella enteritidis typically found?
In the enteric tract of humans and animals (chicken and domestic livestock).
How is Salmonella transmitted?
Via the fecal-oral route.
What condition is caused by Salmonella enteritidis and Salmonella typhimurium, and what is its incubation period?
Condition: Enterocolitis
Incubation Period: 12-48 hours
Common Source: Undercooked chicken
What condition is caused by Salmonella typhi, and what are its key features?
Condition: Typhoid fever
Key Features: Vi (virulence) capsular antigen, multiplies in the Peyer’s patches, predilection to the gallbladder
Incubation Period: 5-21 days
What condition is caused by Salmonella choleraesuis, and who is commonly affected?
Condition: Septicemia
Commonly Affected: Patients with sickle cell anemia or cancer
IDENTIFY
Urea positive
Deaminase positive
Motile
Burnt chocolate or chocolate cake odor
Proteus
IDENTIFY
Indole positive
H2S positive
Proteus vulgaris
IDENTIFY
Facultative gram-negative rods
Non-lactose fermenting
Urease positive
Swarming motility
Indole negative
Proteus mirabilis
What type of infection is commonly associated with Proteus mirabilis?
Complicated urinary tract infection (UTI) associated with nephrolithiasis (staghorn calculi).
IDENTIFY
Motile
Lactose fermenters
Mucoid colonies
Associated with contaminated medical devices (e.g., respirators and other medical instruments)
Ingested from water, vegetables, and food products
Enterobacter
IDENTIFY
-Motile, slow lactose fermenters
-Produces red pigment (prodigiosin) especially in S. marcescens
-DNAse, ONPG, and gelatinase positive
-VP positive
-Transmission may be person-to-person but often associated with contaminated medical devices
Serratia
IDENTIFY
Gram-negative
Catalase, oxidase, and indole positive
Non-lactose fermenters
Facultative anaerobes capable of growth at 4-43°C
Yersinia
IDENTIFY
Transmission: Bite of an infected flea,handling infected animals, inhaling infectious droplets, ingestion of undercooked meat, and human-to-human (pneumonic form)
Disease: Bubonic plague
Appearance: Small gram-negative rods with bipolar (safety pin) staining
Y. pestis
IDENTIFY
Habitat: GIT of swine, rodents, and dogs
Transmission: Ingestion of contaminated food or water
Growth: Optimal at room temperature (RT), cold enrichment
Properties: Invasive and toxigenic
Yersinia enterocolitica
IDENTIFY
Habitat: Wild and domesticated animals (rodents, birds, and rabbits)
Transmission: Contact with infected animals or ingestion of contaminated food or water
Disease: Acute mesenteric lymphadenitis and “pseudo tubercles”
Yersinia pseudotuberculosis
