Tuesday, August 16, 2011

Laboratory Diagnosis of Meningitis: Culture

Immediately, part of the CSF deposit seeded heavily on culture media as blood agar, chocolate agar as ( Thayer Martin media ) and incubated in humid air at 37◦C for 24-48 hours plus 5-10% CO2 . Tube of cooked-meat broth and another blood agar plate as Columbia , Brucella or Scheadler should be seeded by deposit and incubated for 2-5 days in an anaerobic
atmosphere when there is a suspicious of anaerobic infection. The culture inspected after overnight incubation for identifying any growth , if no growth appears after overnight incubation , the plates are reincubated for another day and reinspected for growth (Nigrovic et al ., 2004 ) .
The culture method in tuberculous meningitis is a definitive diagnosis but it is limited by the slow growth rate over several weeks. The sensitivity of the culture method decreases in certain samples with a low number of M. tuberculosis. It could be as low as 40 % for CSF, pleural effusion and peritoneal samples (Schluger et al., 2001).
The radiometric method (BACTEC) is based on the measurement of the radioactive CO2 released by metabolism of the radioactive or carbon labeled palmitic acid present in the liquid culture media from mycobacterial growth. This method

reduces the culture time, but still requires 7 – 10 days for positive cultures (Bonington et al, 2000).

Mycobacteria Growth Indicator Tube (MGIT) system is a non-radiometric method, it has an oxygen sensitive fluorescent sensor embedded in silicone base to serve as an indicator of mycobacterial growth. As the actively growing and respiring mycobacteria consume the dissolved O2, the sensor glows indicating mycobacterial growth. This is observed by using an ultra violet lamp with a wave length of 365 nm (Chitra and Prasad , 2001).

Though the use of culture method in leptospirosis confirms diagnosis and also aids in identifying the prevalent serovar, it is rarely used, as it is very tedious, complicated, expensive technically demanding time consuming, requiring prolonged
incubation minimum 1 month before declaring a sample negative and may not be successful (low sensitivity). Additionally they are highly infectious organisms requiring strict biosafety facilities (Dutta & Christopher , 2005).

Blood or CSF culture for diagnosis of brucellosis in patients with primary infections gives excellent sensitivity results but in individuals with previous contact with the microorganism or occupational exposure and symptoms of acute or persistent infection (very frequent in endemic areas) culture gives poor results( Jordi & Miquel , 2004).

The risk of spread of infection, difficulty in culture, time period of more than 6 weeks and positivity of blood or CSF culture in only 50 to 70% patients do not make culture method the investigation of choice for the diagnosis of brucellosis (Kochar et al ., 2000).

Diagnosis of Lyme disease by culture method can be done

but it is difficult as it requires specialized medium not generally available in most microbiological laboratories and prolonged incubation at relatively low temperatures. Even in specialized labs, the number of spirochetes present in the samples are so low that the yield of culture and even PCR testing are low. The best sensitivity reported in CSF culture in Lyme meningitis is only 10 % as spirochetes are so few in number or even none in the aliquot tested. As a result , diagnosis depends heavily on demonstration of specific anti-B.burgdorferi antibody in serum or CSF (Pachner & Steiner , 2007).

Although T. pallidum cannot be grown in culture, there are many tests for the direct and indirect diagnosis of syphilis.Still,
there is no single optimal test. Direct diagnostic methods include the detection of T. pallidum by Dark-field microscopy examination of fluid or smears from lesions, histological examination of tissues or nucleic acid amplification methods such as PCR. Indirect diagnosis is based on serological tests for the detection of antibodies. Serological tests are fall into two categories: nontreponemal tests for screening and treponemal tests for confirmation . All nontreponemal tests measure both immunoglobulin (Ig) G and IgM antiphospholipid antibodies formed by the host in response to lipoidal material released by damaged host cells early in infection and to the lipid from the cell surfaces of the treponeme itself (Sam , 2005).

In diagnosis of CNS candidiasis ,the significance of a positive culture from the CSF may be unclear. Contamination of the CSF sample may occur because of colonization of the skin or when cultures have been taken from external reservoirs that contain CSF. Several non-culture-based methods have been developed for diagnosing invasive fungal infections of the CNS, such as cryptococcal meningitis and CNS aspergillosis. Similarly, a Candida cell wall component, mannan, has been

used as a target for serological tests. Although the detection of circulating mannan was found to be of limited value in the diagnosis of invasive candidiasis, detection of mannan in CSF could be a valuable tool for diagnosing CNS candidiasis (Frans et al ., 2004).

No comments: