A range of laboratory diagnostic methods has been developed to support patient management and disease control. The choice of diagnostic method depends on the purpose for which the testing is done (e.g. clinical diagnosis, epidemiological survey, vaccine development), the type of laboratory facilities and technical expertise available, costs, and the time of sample collection. Ideally, the clinical Diagnosis of the Staphylococcal infection is done as follows:
The specimen or sample for the diagnosis of the causative agent is collected as per the Site and Nature of the lesions which are as follows:
SPECIMEN SITE OF LESION / AFFECTED AREA
PUS Suppurative Lesion
NASAL / THROAT SWAB Upper Respiratory Tract Infection
SPUTUM Lower Respiratory Tract Infection
BLOOD Septicemia or Bacteraemia
URINE Urinary Tract Infection
CEREBROSPINAL FLUID (CSF) Meningitis
FAECES / STOOL Food Poisoning
COLLECTION & TRANSPORT OF THE SPECIMEN
⇒ The samples should be collected in specific sterile containers by taking all the necessary precautions and under strict aseptic conditions to prevent the contamination.
⇒ Sterile Cotton swabs should be used for the collection of the specimens from the sites such as Nose or Perineum and immediately the swabs are transferred to the test tubes containing Normal saline or Peptone water or directly inoculated onto the sterile media plates.
⇒ In case of the Urine specimen, Mid-stream urine should be collected to prevent the contamination of the Normal flora or other microbes that may be present on or near to the urethral opening.
⇒ The blood specimen should be collected in the appropriate culture bottles as per the age of the patient (Children or Adult) and it should contain the Glucose broth &/or Taurocholate broth.
⇒ All the specimens should be transported immediately to the Laboratory and processed to prevent any erroneous results.
⇒ Most commonly, the Gram staining of the specimen is done by preparing a thin smear of the specimen and staining it with the gram stain. If the specimen contains the Staphylococcus sp. it appears as the purple/blue colored (gram-positive) round shape bacteria in grape-like clusters.
⇒ Also, Acid-fast staining and Albert staining can be done in case of suspected infection of Mycobacterium or Corynebacterium and on the specimens that are collected from Respiratory tract i.e. Sputum and Nasal/Throat swab.
CULTURE AND IDENTIFICATION OF STAPHYLOCOCCUS
⇒ Staphylococcus readily grows on lots of artificial medium in the laboratory but most commonly BA (for Hemolysis) and MSA (Selective medium for Staphylococcus) medium are used for cultivating the staphylococcus in the laboratory.
⇒ The specimens are inoculated on the Blood agar medium (BA) and Mannitol Salt Agar (MSA) medium and then incubated at 37°C for 18-24 hours in the Laboratory incubator.
⇒ After the 18 hours of incubation, Culture plates are examined for the colony morphology and other characters.
⇒ In liquid media like nutrient broth or peptone water, uniform turbidity is produced.
In case of Staphylococcus, The gram-stained smear, prepared from the colonies of the cultured specimen, reveals the Gram-Positive cocci that are arranged in Grape-like Clusters under the microscope.
BIOCHEMICAL TESTS FOR THE IDENTIFICATION OF STAPHYLOCOCCUS
There are large no. of biochemical reactions for the staphylococcus but a few are required for the clinical identification of Staphylococcus in the diagnostic laboratory which is as follows:
⇒ Catalase Test:-
All the Staphylococci whether they are pathogenic or non-pathogenic are catalase positive. This test is mainly done to distinguish the Staphylococcus from the Streptococcus (which is catalase negative).
⇒ Coagulase Test:-
This test is done for distinguishing the Staphylococcus aureus from other Staphylococci which important for differentiating pathogenic strain from the non-pathogenic strains of Staphylococci.
⇒ Mannitol fermentation:-
Staphylococcus aureus is the only Staphylococcus that ferments the Mannitol. This test is of great importance in differentiating pathogenic Staphylococci from the non-pathogenic one.
This test is not commonly done but is important from the diagnostic point of view as to differentiate Staphylococcus aureus from other Staphylococci.
⇒ Staphylococci produce a no. of enzymes but the Presence or absence of DNAase and Phosphatase enzymes are important for distinguishing Staphylococcus aureus from other staphylococci.
THIS IS ALL ABOUT THE LABORATORY DIAGNOSIS OF STAPHYLOCOCCUS AUREUS….