Microbiologic analysis of cerebrospinal fluid

Microbiological research

Microscopy of a stained smear of cerebrospinal fluid allows to identify microbial flora in 10-20% of patients with bacterial meningitis. In bacterial meningitis, meningo-, bean-shaped diplococci located intracellularly in the cytoplasm of neutrophils, or pneumococci, which are also diplococci, but have a triangular shape, and a pair of cocci form a rhombus (covered with a capsule, located extracellularly), can be detected. In some cases, spirochetes, rod-shaped bacteria, and yeast-like fungi cells are visually detected. The data obtained by microscopy are approximate and are confirmed by other methods. The flotation method is used to detect mycobacteria tuberculosis. To isolate the culture of the pathogen, cerebrospinal fluid is seeded on nutrient media. The results of the study depend on the correct collection and transportation of cerebrospinal fluid, the quality of the nutrient media. The frequency of isolation of the pathogen culture is twice as high if the patient has not received antibacterial drugs before the spinal puncture. In practice, it is possible to isolate the pathogen culture from the cerebrospinal fluid in 30-50% of patients with purulent meningitis. Determination of the sensitivity of the isolated culture to antibacterial drugs used to treat meningitis (benzylpenicillin, ampicillin, oxacillin, ceftriaxone, pefloxacin, ciprofloxacin, vancomycin, rifampicin, gentamicin) is mandatory. Fungal cultures are tested for sensitivity to antifungal drugs.

The main rule on which microbiological studies of cerebrospinal fluid are based is that the number of colonies grown depends on the number of seeded microorganisms and their viability at the time of seeding. This means that the volume of cerebrospinal fluid sent for microbiological examination and the speed of its delivery directly affect the results of the study. Particularly large volumes of fluid should be sent for seeding in case of fungal infections, since the concentration of fungal microorganisms in it is extremely low. Thus, when examining a patient with chronic meningitis, the minimum volume of cerebrospinal fluid sent for seeding should be 15-20 ml. Another rule of microbiological examination is the mandatory Gram staining in acute bacterial meningitis. The staining procedure takes about 5 minutes and has a high sensitivity and specificity. The results of this staining allow for the immediate selection of adequate antibacterial therapy. Antibiotics given before lumbar puncture may damage bacterial membranes and thus significantly reduce the specificity of Gram stain, but even then it is useful. In addition to culture, Gram stain, Mycobacterium tuberculosis stain, and India ink stain for cryptococci, a number of serologic tests for viral, bacterial, and fungal antigens are used. Thus, if neurosyphilis is suspected, CSF and RIF should be tested.

Virological methods

The isolation of virus culture is usually used only for scientific purposes.

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