evaluation of microbial D- and L-lactate production as biomarkers of infection.

Mammalian cells produce and metabolize almost exclusively L-lactate, bacterial species have the capacity to produce both D-lactate and L-lactate. The aim of this study was to evaluate the intrinsic production of D- and L-lactate in the most common pathogenic microorganisms causing septic arthritis (SA) and periprosthetic joint infection (PJI) as a potential biomarker for the diagnosis of infection. Following microorganisms were grown according to ATCC culture guides and tested for production of D- and L-lactate: (ATCC 43300), (ATCC 35984), (ATCC 19433), (ATCC 19615), (ATCC 25922), (ATCC 27853), (ATCC 11827), and (ATCC 90028). Pathogens were inoculated in 8 ml of appropriate liquid media and incubated as planktonic or biofilm form in either aerobic, anaerobic or CO atmosphere up to 312 h. D- and L-lactate measurements were performed at different time points: 0, 6, 9, 12 and 24 h, then once per day for slow-growing pathogens. Samples were serially diluted and plated for colony counting. Liquid culture media without microorganisms served as a negative control. Production of D-lactate was observed in all tested microorganisms, whereas no L-lactate was detected in , and . Maximal concentration of D-lactate was produced by (10.99 mmol/L), followed by (1.22 mmol/L), and (0.48 mmol/L). Maximal L-lactate concentration was observed in (10.12 mmol/L), followed by (9.71 mmol/L), (2.64 mmol/L), and (2.50 mmol/L). bacterial biofilm produced significantly higher amount of D- and L-lactate compared to planktonic form ( = 0.015 and  = 0.002, respectively). Our study has demonstrated that the most common pathogenic microorganisms causing SA and PJI have the capability to generate measurable amounts of D-lactate in both planktonic and biofilm form, highlighting the practical value of this biomarker as an indicator for bacterial and fungal infections. In contrast to D-lactate, the absence of L-lactate production in certain tested bacteria, as well as in fungi, suggests that L-lactate is not eligible as a biomarker for diagnosing microbial infections.