Comparison of two methods for detection of mollicutes (Mycoplasmatales and Acholeplasmatales) in cell cultures in the Netherlands

Application of cell culture enrichment for improving the sensitivity of mycoplasma detection methods based on nucleic acid amplification technology (NAT)

Herein, we present data demonstrating that the application of initial cell culture enrichment could significantly improve mycoplasma testing methods based on the nucleic acid amplification technology (NAT) including a polymerase chain reaction (PCR)/microarray method. The results of the study using Vero cells demonstrated that this cell culture is able (1) to support efficient growth of mycoplasmas of primary interest, i.e., species found to be cell line contaminants, (2) to increase the sensitivity of NAT assay to the detection limits of the conventional broth/agar culture methods, and (3) to reduce the time required for mycoplasma testing fourfold in comparison with the conventional methods. Detection and identification of mycoplasmal agents were conducted using a modified PCR/microarray assay based on genetic differences among Mollicutes in the 16S-23S rRNA intergenic transcribed spacer (ITS). The application of nano-gold/silver enhancement technology instead of previously used fluorescent dyes significantly simplified the readout of microarray results and allowed us to avoid using expensive scanning equipment. This modification has the potential to expand the implementation of microarray techniques into laboratories involved in diagnostic testing of mycoplasma contamination in cell substrates and potentially in other biological and pharmaceutical products.

Colistin sulfate as a suitable substitute of thallium acetate in culture media intended for mycoplasma detection and culture

Thallium acetate in concentrations of 500 to 1000 mg/l is tolerated in the culture by the most mollicutes of the orders Mycoplasmatales and Acholeplasmatales and by this reason it is added in the culture media as a selective element for the detection and propagation of mycoplasmas and acholeplasmas. Because of the high toxicity of thallium acetate and its accumulation in the environment, thallium acetate is not biodegradable, an alternative was searched. The results and analysis of tests with nine mollicute species are presented here. It is recommended to replace thallium acetate in the formulations where it is used and colistin sulfate is proposed as its substitute.

Application of a Mycoplasma group-specific PCR for monitoring decontamination of Mycoplasma-infected Chlamydia sp. strains

Mycoplasma contamination of biological materials remains a major problem. Most contaminations are caused by the use of Mycoplasma-contaminated cell lines. We adapted a Mycoplasma group-specific PCR to detect Mycoplasma contamination in cell lines and demonstrate its use in monitoring decontamination procedures with Mycoplasma-contaminated suspensions of Chlamydia spp. Three different methods were investigated: the use of Mycoplasma-specific antiserum in cell culture, physical separation by the combined use of enzymatic treatment and differential centrifugation, and the use of detergents. With these methods only incubation with Triton X-100 resulted in decontamination of Mycoplasma-contaminated suspensions of several laboratory strains of Chlamydia pneumoniae, C. pecorum, and C. trachomatis. Only one C. pneumoniae strain, UZG-1, was sensitive to Triton X-100 treatment. Since 39 of 40 throat swabs from patients with symptoms of an upper respiratory tract infection had positive reactions in the Mycoplasma group-specific PCR, this procedure could also have clinical significance in attempts to propagate C. pneumoniae strains from clinical specimens.

Rapid, sensitive PCR-based detection of mycoplasmas in simulated samples of animal sera

A fast and simple method to detect mycoplasmal contamination in simulated samples of animal sera by using a PCR was developed. The following five mycoplasma species that are major cell culture contaminants belonging to the class Mollicutes were investigated: Mycoplasma arginini, Acholeplasma laidlawii, Mycoplasma hyorhinis, Mycoplasma orale, and Mycoplasma fermentans. After a concentration step involving seeded sera, genus-specific primers were used to amplify a 717-bp DNA fragment within the 16S rRNA gene of mycoplasmas. In a second step, the universal PCR was followed by amplification of variable regions of the 16S rRNA gene by using species-specific primers, which allowed identification of contaminant mycoplasmas. With this method, 10 fg of purified DNA and 1 to 10 color-changing units of mycoplasmas could be detected. Since the sensitivity of the assay was increased 10-fold when the amplification products were hybridized with an internal mycoplasma-specific 32P-labelled oligonucleotide probe, a detection limit of 1 to 10 genome copies per PCR sample was obtained. This highly sensitive, specific, and simple assay may be a useful alternative to methods currently used to detect mycoplasmas in animal sera.

Survival of Mycoplasma hyorhinis in trypsin solutions

The survival of four strains of Mycoplasma hyorhinis in stock solutions of trypsin was tested at 22, 4 and -15 degrees C. Low (10(4)-10(5) cfu/ml) and high (10(6)-10(7) cfu/ml) initial concentrations of each strain were used, each was tested three times. A regular decrease of low and high concentrations (1 log in 10 and 20 min, respectively) was seen at 22 degrees C. At 4 degrees C the low concentrations showed a reduction of about 1 log/h, while apart from one strain high concentrations hardly decreased during the first 6 h and the survival time ranged from 24 to more than 30 h at the end of which there was a reduction of 4 logs. At -15 degrees C low concentrations survived up to 1 week in only one of the three tests, high concentrations survived for more than 12 weeks (reduction 3 logs). These latter results suggest that mycoplasmas may be present in trypsin as clumps, which deteriorate very slowly. A study was also performed to compare the sensitivity of different cultural procedures for detecting mycoplasmas.

Microbiological cultivation of Mycoplasma hyorhinis from cell cultures

The failure of many cell culture isolates of Mycoplasma hyorhinis to grow on microbiological media has stressed the need for alternate assays to detect these organisms. The use of freshly prepared yeast extract in mycoplasmal media together with incubation in 5% CO2/air successfully detected M. hyorhinis in 12 of 12 infected cultures. These were not detected by the use of conventional mycoplasmal media using aerobic or anaerobic incubation. This assay may also be helpful in detection of other mycoplasmal species commonly isolated from cell cultures.

AdoP assay detection of mycoplasmal contamination in biological media

The AdoP assay has been shown to be reliable in controlling the absence of mycoplasmas in filtered sera. We therefore compared its effectiveness with that of other tests in the detection of animal cell contamination by mycoplasmas.

Indicator cell lines for the detection of hidden mycoplasma contamination, using an adenosine phosphorylase screening test

Mycoplasmas are a major cause of cell culture contamination and are especially troublesome during HAT selection. The enzyme adenosine phosphorylase (adoP) is present in all common mycoplasma species but is considered to have a low activity in mammalian cells. However, using an adoP screening test, we have observed that some cell cultures do possess an intrinsic adoP activity leading to false positive results. Moreover, as a false negative result, we encountered a variant of Mycoplasma orale (identified after cultivation on agar and immunostaining) which was not detectable with the adoP screening in cell culture supernatants and only at low levels in cell lysates. To increase the low signal/noise adoP ratio found there, we used an indicator cell line with low intrinsic activity. Indicator cells were inoculated with the test supernatant and the adoP activity of these infected cells were measured after lysis. The procedure diminished the effect of biological variation in intrinsic enzyme activity between the several cell lines tested. Furthermore, in another mycoplasma infected cell line (with M. fermentans), this infection was only reliably detected using these indicator cells. With this procedure we obtained rapid results which were concordant with those obtained using the time consuming cultivation on agar.

Survey of Mycoplasma infections in cell cultures and a comparison of detection methods

A total of 1424 cell cultures was assayed for mycoplasmas by microbiological culture and fluorescent DNA staining. Of these cultures, 412 (29%) were infected with mycoplasmas. The most frequently occurring mycoplasma species were Mycoplasma orale (34%), M. hyorhinis (26%), M. arginini (21%), M. fermentans (13%) and Acholeplasma laidlawii (5%). A few isolates each of M. hominis, M. pulmonis and M. bovis were also detected. When detection methods were compared, microbiological culture produced false-negative results for 0.7% (3 of 412) of the infected cell cultures. DNA staining performed directly on the cells was falsely negative in 2.4% (5/207) of the mycoplasma-infected cultures that were compared, DNA staining performed on indicator cells was falsely negative in 3.1% (7/226). False positives appeared in direct DNA-staining in 1.8% (7/386) of the mycoplasma-free cultures and with DNA staining on indicator cells in 0.5% (3/620). For 11% of the cell cultures, the reading of the DNA staining was ambiguous. With DNA staining on indicator cells, 10% of the test results were ambiguous, but by further passage and staining on new indicator cells it was possible to get a definite diagnosis.