Analysis of Infection Loads in Mycoplasma genitalium Clinical Specimens by Use of a Commercial Diagnostic Test

Abstract

Mycoplasma genitalium is a common sexually transmitted infection with a propensity to acquire resistance to commonly used antimicrobial therapies. Bacterial load has been linked to patient symptoms and the success of treatment. In this study, we demonstrate methodology to estimate load from routine diagnostic assays using the ResistancePlus MG test (SpeeDx Pty Ltd., Australia). The method gave comparable quantitation to an M. genitalium-specific 16S rRNA quantitative PCR (qPCR; Spearman r = 0.94) for the samples analyzed (n = 499, including urine and swab types as detailed below) and was, therefore, employed to analyze typical load levels for samples in a diagnostic laboratory (total of 1,012 tests). When stratified by sample type, female urine (median, 826 genomes/ml) had the lowest load. This was significantly lower than median loads for all other sample types (male urine [6.91 × 10³ genomes/ml], anal swabs [5.50 × 10³], cervical swabs [8.15 × 10³], endocervical swabs [3.97 × 10³], and vaginal swabs [6.95 × 10³]) (P < 0.0001). There were no significant differences in load estimates between the other sample types. Reproducibility of load estimates conducted on the same samples was high (r > 0.85). In conclusion, this methodology to provide load estimates for M. genitalium can be easily integrated into routine diagnostic laboratory workflow. Given the association between organism load, symptoms, and treatment success, load assessment has future diagnostic potential.

Keywords: Mycoplasma genitalium; diagnostic test; infection load; sample type; urine.

FIG 1

Overview of sample collection, data extraction, and analysis. (A) For 531 diagnostic tests, data extraction was performed from the routine diagnostic ResistancePlus MG assay. A 16S rRNA qPCR was performed on a subset of 499 samples for a comparison of load estimation methods. (B) Data were extracted for routine diagnostic ResistancePlus MG assay for an additional 481 samples and combined with the results for the 531 samples (from above), making a total of 1,012 samples.

FIG 2

Comparison of load estimation by 16S rRNA qPCR and ResistancePlus MG assay. Sample loads (genome equivalents per 5-μl sample extract) were graphed in a scatter plot with linear regression.

FIG 3

Diagnostic specimen sample loads stratified by sample type. Sample loads estimated from the ResistancePlus MG assay (expressed as genome equivalents per swab or per 1 ml for urine) were log₁₀ transformed and plotted. Analysis of the range and distribution of loads is presented in tabular form. Whiskers represent the 10th/90th percentiles, and outliers are indicated. Samples types not represented in the figure include urethral swabs (n = 7), rectal swab (n = 1), and urine samples where sex was indicated as “other” (n = 2). Only the median for female urine samples was significantly different to other sample types (P < 0.0001).

FIG 4

Analysis of the reproducibility of load estimates. Sample load estimates for the primary test (log transformed, n = 39 samples) were separately compared with two repeat analysis tests by scatter plot with linear regression.