The Salmonellae: Morphology and Identification, Classification, Pathogenesis and Clinical Findings, the “Enteric Fevers”, Enterocolitis. Diagnostic Laboratory

Question 1

Morphology and identification of Salmonella

Answer 1

Salmonella are Gram-negative, motile, facultative anaerobic rods with peritrichous flagella. They are non-lactose fermenters and produce hydrogen sulfide (H2S), which gives colonies a black center on selective media like SS agar or Hektoen enteric agar. On MacConkey agar, they form pale colonies due to lack of lactose fermentation. They are oxidase-negative and catalase-positive. Identification involves biochemical testing and serotyping.

Question 2

Classification of Salmonella

Answer 2

Salmonella are classified into two species: Salmonella enterica and Salmonella bongori. Most human pathogens are in Salmonella enterica, which is divided into six subspecies. The most relevant is S. enterica subspecies enterica, which includes >2500 serovars (serotypes). Classification is based on O (somatic), H (flagellar), and Vi (capsular) antigens. Important serovars include S. Typhi, S. Paratyphi, S. Typhimurium, and S. Enteritidis.

Question 3

Pathogenesis and clinical findings of Salmonella

Answer 3

Infection occurs via ingestion of contaminated food or water. Salmonella invades intestinal epithelial cells using Type III secretion systems. It triggers inflammatory responses and survives in macrophages. Symptoms include nausea, vomiting, abdominal cramps, diarrhea (often non-bloody), and fever. The incubation period is 6–48 hours. Severity depends on serovar, host immunity, and age. In immunocompromised hosts, bacteremia and focal infections can occur.

Question 4

Enteric fevers caused by Salmonella

Answer 4

Enteric fevers (e.g., typhoid and paratyphoid fever) are systemic illnesses caused by S. Typhi and S. Paratyphi. After ingestion, bacteria invade the intestinal mucosa, multiply in macrophages, and spread via the bloodstream. Symptoms include prolonged fever, abdominal pain, hepatosplenomegaly, rose spots, and constipation or diarrhea. Complications include intestinal perforation and hemorrhage. Chronic carriers may excrete bacteria in stool for months or years.

Question 5

Enterocolitis caused by Salmonella

Answer 5

Salmonella enterocolitis (gastroenteritis) is caused by non-typhoidal serovars like S. Typhimurium or S. Enteritidis. It presents with sudden onset of nausea, vomiting, abdominal cramps, fever, and diarrhea (which may be watery or bloody). It is usually self-limited, lasting 3–7 days. It results from invasion of the mucosa and inflammation rather than toxin production.

Question 6

Diagnostic laboratory tests for Salmonella

Answer 6

Diagnosis is by stool culture on selective media (e.g., SS agar, Hektoen enteric agar, XLD agar). Blood cultures may be positive in enteric fever. Biochemical tests confirm identity, including H2S production, urease negativity, and citrate utilization. Serotyping with antisera for O, H, and Vi antigens is used for strain identification. PCR and molecular methods may be used for rapid detection in some settings.

Question 7

Treatment of Salmonella infections

Answer 7

Uncomplicated enterocolitis usually does not require antibiotics. For severe disease or high-risk patients, ciprofloxacin, azithromycin, or third-generation cephalosporins (e.g., ceftriaxone) are used. For enteric fever, treatment includes ceftriaxone, azithromycin, or fluoroquinolones. Antibiotic resistance is increasing; susceptibility testing is essential. Chronic carriers may need prolonged treatment with antibiotics like ciprofloxacin.

Question 8

Epidemiology of Salmonella

Answer 8

Salmonella infections are a major global cause of foodborne illness. Non-typhoidal Salmonella is common in undercooked poultry, eggs, dairy, and produce. S. Typhi and S. Paratyphi are transmitted via fecal-oral route and are endemic in developing countries. Human carriers and poor sanitation contribute to spread. Animals (especially reptiles and poultry) are reservoirs for non-typhoidal strains.

Question 9

Prevention and control of Salmonella

Answer 9

Prevention includes proper food handling, cooking, and storage; hand hygiene; and improved sanitation. Travelers to endemic areas should avoid untreated water and raw foods. Two vaccines are available for typhoid fever: an oral live attenuated vaccine (Ty21a) and an injectable Vi capsular polysaccharide vaccine. There is no vaccine for non-typhoidal Salmonella. Identification and treatment of chronic carriers is important in controlling typhoid.