DNA probes and PCR in diagnosis of mycoplasma infections

Laboratory diagnosis of mycoplasma infections is hampered by the difficulty or total failure to cultivate the organisms in vitro, and by the frequently weak and poorly specific serological response of the host. DNA probes consisting of cloned ribosomal RNA genes, cDNA to mycoplasmal rRNA, synthetic 16S rRNA oligonucleotide sequences, or cloned mycoplasmal protein genes, have been developed and applied as diagnostic tools in a variety of human and animal mycoplasma infections. These included primary atypical pneumonia caused by Mycoplasma pneumoniae, urogenital infections associated with M. genitalium and Ureaplasma urealyticum, and infections with M. fermentans, M. penetrans or M. pirum--mycoplasmas recently incriminated as cofactors in AIDS. DNA probes were also designed to aid in diagnosis of mycoplasma diseases of farm and laboratory animals, and the hard-to-diagnose mycoplasma infections of cell cultures. Sensitivity of mycoplasma detection by the different probes ranged between 10(3) and 10(6) colony-forming units, a level which may not be sufficiently high for use in a clinical laboratory. The introduction of PCR has pushed aside the previously developed DNA probes, by providing faster and much more sensitive tests. The sensitive level of a PCR test can be as low as a single organism, enabling detection of mycoplasmas in patients treated with antibiotics and in asymptomatic patients. PCR becomes positive prior to serological response and is also effective in immunocompromised hosts. PCR was shown to be most valuable in detection and identification of the non-culturable plant and insect mycoplasma-like organisms (MLOs). Nevertheless, false-negative PCR results are rather common due to inhibitors of the PCR reaction in the clinical specimen, while false-positive results may occur due to contamination of the reagents with target DNA. In conclusion, the PCR procedure is still too complex to be carried out in a routine diagnostic laboratory. PCR prepackaged quality-controlled diagnostic kits are now in the process of rapid development. Once these kits become available, and at a reasonable cost, PCR will certainly take its place as a major diagnostic tool in the routine diagnosis of mycoplasma infections.