Salmonella species commonly cause infections of the intestines that are characterized by diarrhea, fever and abdominal cramps. The disease spreads due to consumption of contaminated water or foods such as eggs and meat. Fatal complications can develop if the salmonella infections spread beyond the intestines to the bloodstream. In cases with strong clinical suspicion, physicians may order some tests to confirm the diagnosis.
Sample Collection
Patient's feces, collected in a screw-capped plastic bottle, is the most common specimen used to diagnose salmonella infection. Mackie and McCartney's Manual of Practical Medical Microbiology suggests that if there is a delay in the transport of the sample and the weather is warm, the sample should be placed in a solution of glycerol and saline and transported to the laboratory. If the doctor suspects that the infection has spread beyond the intestines, a blood, pus, bile or urine sample may be recommended for isolation of the bacteria.
Culture
Culture is a test performed in a microbiology laboratory to specifically isolate and identify pathogenic bacteria such as salmonella from patient specimens. A small amount of the specimen is inoculated into a growth-medium plate, which contains gels that promote the growth of bacteria. The culture plates are incubated at 37 degrees Celsius for 18 hours. The culture plates are observed for the presence of bacterial colonies, and these colonies are further tested using biochemical tests to specifically identify the organism. According to The New York Times Health Guide, presence of normal fecal bacteria indicates lack of salmonella infection, and abnormal results may indicate intestinal infection. Presence of even small amounts of salmonella strains in blood or urine indicate infection, as these fluids are otherwise sterile.
Biochemical Tests
Biochemical tests are confirmation tests that help identify the specific species and strains of Salmonella species by observing their abilities to ferment certain sugars, decarboxylate amino acids and utilize citrate. The Mackie and McCartney's Manual of Practical Medical Microbiology recommends a combination of these tests for confirmation, especially if the bacterium is not salmonella.
Antimicrobial Assays
Antimicrobial assays are performed to test the antibiotic susceptibility pattern of salmonella strain that has been isolated during culture. A small portions of the salmonella colonies are inoculated on a growth-medium plate. Small paper discs impregnated with antibiotics are placed on the plate, and the plate is incubated for 18 hours at 37 degrees Celsius. As per Mackie and McCartney's Manual of Practical Medical Microbiology, the common antibiotics used for salmonella include chloreamphenicol, ampicillin, gentamicin, tetracycline and cotrimoxazole. A clearing around the disc indicates that the antibiotic is effective against salmonella strain isolated.
Serological Tests
Although, culture is a reliable test to diagnose salmonella infections, the results take more than 48 hours. Serological tests such as agglutination tests can give results in 1 to 2 hours and serological tests are as reliable as culture in diagnosing salmonella infections such as typhoid fever.
A small amount of serum is obtained from the patient's blood sample and are mixed with samonella antigens on a slide and rotated. Formation of clumps indicates a positive test.
References
- "Mackie and McCratney's Manual of Practical Medical Microbiology"; J. G. Collee, A. G. Fraser, B. P. Marmionand A. Simmons; 1996
- The New York Times Health Guide: Salmonella Enterocolitis
- "Indian Journal of Medical Sciences"; P Sugandhi Rao, S Vishnu Prasad, G Arunkumar, PG Shivananda; 1999
