Salmonella, Shigella, Pseudomonas and Legionella Flashcards
What are the key characteristics of Enterobacteriaceae?
◾ Small, straight, Gram-negative rods
◾ Found in soil, water, decaying matter, and intestines of humans/animals
◾ Non-spore forming
◾ Mostly motile (except Shigella, Klebsiella, Yersinia)
◾ Facultative anaerobes
◾ Oxidase (-), Catalase (+)
◾ Ferment glucose, reduce nitrates to nitrites
◾ Use anaerobic pathway to ferment carbohydrates
Further notes:
Facultative anerobes, while capable of aerobic respiration, may not always use cytochrome c oxidase. They can utilize other electron transport chain components.
What are the four metabolic characteristics common to all Enterobacteriaceae?
◾ Facultative anaerobes
◾ Ferment glucose
◾ Oxidase negative
◾ Reduce nitrates to nitrites
What are the general features of Salmonella?
◾ Gram-negative rods
◾ Peritrichous flagella (motile)
◾ Uncapsulated (except S. typhi)
◾ Non-spore forming
What are the three main antigens of Salmonella?
✔ O antigen (heat-stable LPS, outer membrane, serotype variation)
✔ H antigen (flagellar, diphasic variation, heat-labile)
✔ Vi antigen (capsule, found in S. typhi)
Where are S. enterica and S. bongori found?
S. enterica: Digestive tracts of humans, animals, environment
S. bongori: Cold-blooded animals (e.g., reptiles)
How are Salmonella serotypes categorized clinically?
✔ Typhoidal Salmonella: Human-to-human, causes typhoid/paratyphoid fever
✔ Non-Typhoidal Salmonella (NTS): Animal-to-human and human-to-human, causes gastroenteritis
How does Salmonella evolve?
✔ Horizontal gene transfer
✔ New serovars arise due to pathogenicity islands
How is Salmonella transmitted?
◾ Fecal-oral route
◾ Contaminated food/water
◾ Common sources: poultry, eggs, beef, milk, vegetables
How does Salmonella cause infection?
◾ Ingested bacteria survive stomach acid (acid-tolerance genes)
◾ Invade intestinal mucosa
◾ Can spread to liver, spleen, bone marrow
◾ Triggers inflammatory response
What are the main virulence factors of Salmonella?
✔ Enterotoxin (causes diarrhea)
✔ Endotoxin (LPS layer) (causes fever)
✔ O antigen (potent stimulator of host’s immune system)
✔ Vi capsule (inhibits complement)
✔ Flagella (helps adhesion and motility)
How does Salmonella survive inside macrophages?
◾ It prevents the normal fusion of the Salmonella-Containing Vacule with lysosomes, which contain potent degradative enzymes
◾ Resists oxidative burst
◾ Uses RecA and RecBC proteins for DNA repair
◾ mgtCBR operon reduces flagellin expression to evade immune response
How does typhoidal Salmonella spread in the body?
◾ Bacteria enter the lymphatic system → bloodstream
◾ Infects organs (liver, spleen, kidneys)
◾ Endotoxins cause vascular, nervous, gastrointestinal damage
◾ Leads to hypovolemic shock, septic shock
How does non-typhoidal Salmonella cause disease?
◾ Non-invasive NTS: Food poisoning, requires high infectious dose
◾ Invasive NTS: Enters blood, causes bacteremia, sepsis
What are the five clinical patterns of Salmonella infection?
(1) Gastrointestinal tract infection (diarrhea)
(2) Enteric fever (typhoid/paratyphoid)
(3) Bacteremia (spreads in blood)
(4) Metastatic infection (joints, CNS, bones)
(5) Chronic carrier state
How is Salmonella diagnosed in enterocolitis vs. enteric fever?
◾ Enterocolitis: Stool culture
◾ Enteric fever: Blood culture, bone marrow culture
What selective media are used for Salmonella stool cultures?
✔ Desoxycholate citrate agar (DCA)
✔ Xylose lysine desoxycholate agar (XLD)
✔ Salmonella-Shigella agar (SS)
What biochemical tests confirm Salmonella?
✔ Triple Sugar Iron (TSI) test: Alkaline slant, acid butt, gas, H₂S production
✔ Slide agglutination test: Detects O and H antigens
What vaccines are available for Salmonella?
Ty21a: Live, oral vaccine.
Vi polysaccharide: Inactivated, injectable vaccine.
What are key public health measures to prevent Salmonella infections?
(1) Availability of treated water
(2) Proper disposal of human waste
(3) Prevention of contamination by food handlers and rodents.
What food safety practices help prevent Salmonella?
(1) Raw animal-origin foods must not contact cooked foods
(2) Proper refrigeration of cooked foods
(3) Handwashing after handling pets.
What is the initial symptom of Shigella infection?
Abdominal colic (cramping pain).
What factors influence the severity of Shigella infection?
The species involved and the patient’s age.
What is the most severe complication of Shigella dysenteriae infection?
Hemolytic uremic syndrome (HUS), characterized by:
◾ Hemolytic anemia
◾ Acute kidney failure (uremia)
◾ Low platelet count (thrombocytopenia).
Does Shigella cause septicemia?
Rarely, because it usually remains localized in the intestine.
How does Shigella appear on gram stain?
gram-negative bacilli.
What is the lactose fermentation status of Shigella on MacConkey agar?
Non-lactose fermenter (except Shigella sonnei, which is a late lactose fermenter).
What is the typical Triple Sugar Iron (TSI) test result for Shigella?
Alkaline slant, acid butt, no gas, no H₂S.
Further notes:
Triple Sugar Iron test: This is a microbiological test used to determine the ability of a microorganism to ferment sugars and to produce hydrogen sulfide. It is often used to differentiate enteric bacteria, including Salmonella and Shigella.
Components of TSI agar:
◾ Three sugars: glucose (0.1%), sucrose (1%) and lactose (1%).
◾ pH indicator: phenol red (red in alkaline conditions, yellow in acidic conditions)
◾ Hydrogen sulfide indicator: ferrous sulfate (reacts with hydrogen sulfide to produce a black precipitate)
Interpretation of results:
◾ If the microorganism can ferment any of the three sugars, the pH indicator will turn yellow, indicating acid production.
◾ The slant and butt of the agar may both turn yellow if lactose or sucrose are fermented.
◾ If only glucose is fermented, the slant may revert to red (alkaline) after 24 hours due to the oxidation of the acid produced.
◾ If the microorganism produces hydrogen sulfide, a black precipitate will form in the butt of the agar.
◾ Gas production is indicated by bubbles or cracks in the agar.
◾ [6-minute video]
What is the primary treatment for most Shigella infections?
Supportive therapy with oral rehydration; antibiotics are usually not required.
How can Shigella infections be prevented?
Proper sewage disposal, safe food handling, good personal hygiene, and adequate water chlorination.
Briefly discuss Shigella culturing.
🧫 MacConkey agar: On MacConkey agar, Shigella colonies appear colorless due to their inability to ferment lactose.
🧫 Hektoen Enteric (HE) agar: Shigella forms green colonies on HE agar. [Image]
🧫 Xylose Lysine Deoxycholate (XLD) agar: Shigella forms red colonies on XLD agar.
What type of infections does Pseudomonas aeruginosa commonly cause?
Nosocomial (hospital-acquired) infections.
What is the natural habitat of Pseudomonas aeruginosa?
Moist environments such as sink drains, vegetables, river water, and antiseptic solutions [due to its resistance to many disinfectants]
At what temperatures can Pseudomonas aeruginosa survive?
4–42°C (optimal growth between 36–42°C).
How is Pseudomonas aeruginosa acquired?
Mostly from the environment; occasionally patient-to-patient spread.
What is the distinctive odor of Pseudomonas aeruginosa?
A sweet, fruity smell.
What are the three stages of Pseudomonas aeruginosa infection?
(1) Attachment and colonization
(2) Local invasion
(3) Dissemination and systemic disease.
What major virulence factors does Pseudomonas aeruginosa produce?
Capsule (antiphagocytic), Pili (adhesion), Lipopolysaccharide (endotoxin), Pyocyanin (impairs ciliary function).
What are key toxins and enzymes of Pseudomonas aeruginosa?
Exotoxin A (inhibits protein synthesis), Exotoxin S (disrupts leukocyte function), Leukocidin (kills WBCs), Elastase (damages tissue with elastin), Phospholipase C (heat-labile hemolysin).
Who is at high risk for Pseudomonas infections?
Burn patients, cancer patients (chemotherapy-induced neutropenia), ventilated patients, cystic fibrosis patients.
What infections does Pseudomonas aeruginosa cause in immunocompetent individuals?
Osteochondritis after puncture wounds, hot tub folliculitis, swimmer’s ear (acute otitis externa), contact lens-associated conjunctivitis.
What infections does Pseudomonas aeruginosa cause in immunocompromised individuals?
Malignant otitis externa in diabetics, meningitis after trauma or surgery, sepsis and meningitis in newborns, endocarditis or osteomyelitis in IV drug users, pneumonia in bronchiectasis patients.
Malignant otitis externa in diabetics, meningitis after trauma or surgery, sepsis and meningitis in newborns, endocarditis or osteomyelitis in IV drug users, pneumonia in bronchiectasis patients.
Aerobic conditions at 37°C.
How does Pseudomonas aeruginosa appear on MacConkey agar?
Non-lactose fermenter.
What are key biochemical test results for Pseudomonas aeruginosa?
Oxidase-positive, Catalase-positive, Urease-negative, TSI: alkaline slant, alkaline butt, no gas, no H₂S.
What type of bacterium is Legionella?
A gram-negative bacillus commonly found in water sources.
How many species and serogroups of Legionella have been identified?
50 species and 70 serogroups.
What is the most important species of Legionella in human disease?
Legionella pneumophila.
How does Legionella pneumophila appear microscopically?
Thin, pleomorphic, rod-like formations.
What structural features does Legionella possess?
Inner and outer membranes, Pili (fimbriae) for adherence, and a single polar flagellum for motility.
What nutrients are required for Legionella growth?
Iron, L-cysteine, and Biofilm (protozoan association preferred).
here is Legionella commonly found?
Non-marine aquatic environments such as lakes and ponds.
Where does Legionella thrive best?
Areas with high concentrations of rust, algae, and organic particles.
How is Legionella transmitted?
Through inhalation of aerosolized water droplets containing the bacteria
What are common sources of Legionella contamination?
Air conditioning cooling towers, showerheads, pipes, whirlpools, humidifiers, grocery store misters, and respiratory therapy devices.
What type of parasite is Legionella?
A facultative intracellular parasite.
How does Legionella infect human cells?
Binds to alveolar macrophages, engulfed into a phagosomal vacuole, prevents lysosomal fusion, multiplies in the phagosome, causes cell lysis.
What are the two main diseases caused by Legionella?
Legionnaire’s disease and Pontiac fever.
What are the symptoms of Legionnaire’s disease?
Fever, chills, dry cough, diarrhea, nausea, and pneumonia.
What are the symptoms of Pontiac fever?
Fever, headache, severe muscle aches; self-limiting and does not cause pneumonia.
What factors suggest a Legionella infection?
History of respiratory tract infection and radiological findings of pulmonary infiltrates.
What culture medium is used to isolate Legionella?
Buffered charcoal-yeast extract (BCYE) agar
What are the key characteristics of Legionella pneumophila?
Gram-negative bacillus, fastidious organism, replicates in macrophages, cell-mediated immunity is primary defense, transmitted via aerosol.
Who is at the highest risk for Legionella infection?
Middle-aged and older individuals, especially those with COPD, cancer, diabetes, dialysis, AIDS.
