The below mentioned article provides an overview on Laboratory Diagnosis of Bacterial Diseases.
Laboratory diagnosis of Infectious diseases can be done as:
1. Specimen: should be collected aseptically.
2. Demonstration of bacteria and their products.
3. Isolation of bacteria by culture.
4. Serology: Detection of antibody titre.
5. Animal inoculation test.
6. Identification of bacteria by biochemical tests and serological tests.
Quality control of laboratory reagents should be done regulatory.
Specimens should be collected aseptically in sterile container, well-chosen and of good quality. Specimen container should be correctly labelled. Special risk of infection associated with handling of the specimen, if any, should be indicated on the label (e.g. Hepatitis B or HIV infection).
Transport and Storage:
The specimens should be despatched immediately to the laboratory since some specimens (e.g. stool) act as good growth medium for the commensal and non- fastidious bacteria that outgrow the pathogens. Particularly Gram-negative bacilli. Non-sporing anaerobic bacilli and Neisseria sp. are fragile during transport.
Therefore, these specimens are to be inoculated immediately after collection. If delay is expected, they should be collected in transport media. When the delay is unavoidable, storage at 4°C can be done but bacterial overgrowth is the main problem (e.g. urine) found low temperature kills some bacteria (Table 95.1).
2. Detection of Bacterial and their Products:
i. Direct demonstration is a rapid diagnosis method and provides the quality of sample and nature of pathogens. Microscopic examination of (Gram, Ziehl-Neelsen, Albert) stained smears are done.
Many organisms (Legionella pneumophila), Chlamydia trachomatis, rickettsial antigen) can be detected in tissues. Immunofluorescence tests using monoclonal antibodies are highly specific.
iii. Periodic Acid-Schiff (PAS):
And methenamine silver-staining can detect parasites in tissue (Pneumocystis carinii).
The specimen is inoculated in suitable media and incubated aerobically and anaerobically. Isolates are identified by a series of biochemical tests and agglutination tests.
4. Detection of Antibody (Serology):
About 5-10 ml of blood is collected aseptically in a sterile container and allowed to clot. After clotting, serum is separated by centrifugation. Serum is used for serological tests and antibiotic assays. Serum is used for most of the tests, but other body fluids (e.g. pleural, pericardial, cerebrospinal fluid, urine, faecal extracts and saliva) are sometimes used.
Haemolysis and bacterial contamination of sera are not suitable for tests. “Paired sera ” (Samples collected at weekly intervals) are to be tested for detection of rising antibody titre. Antibodies against whole organisms or against only one class of antigen can be determined.
Advantage of serological tests over culture:
(1) Where causative organisms cannot be cultivated, e.g. syphilis.
(2) When the organisms grow slowly in culture.
(3) When the organisms have disappeared from blood and have taken shelter in internal organs.
(4) It takes few hours to perform the test.