A microtitre plate-based assay for the screening of beta-lactams

Staphylococcus aureus cell wall stress stimulon gene-lacZ fusion strains: potential for use in screening for cell wall-active antimicrobials

lacZ fusion strains were constructed using the promoters of five cell wall stress stimulon genes: pbp2, tcaA, vraSR, sgtB, and lytR. All fusion strains were induced only in the presence of cell wall-active antibiotics, suggesting the potential of these strains for use in high-throughput screening for new cell wall-active agents.

Differential assay for high-throughput screening of antibacterial compounds

The previously described Bacillus subtilis reporter strain BAU-102 is capable of detecting cell wall synthesis inhibitors that act at all stages of the cell wall synthesis pathway. In addition, this strain is capable of detecting compounds with hydrophobic/surfactant activity and alternative mechanisms of cell wall disruption. BAU-102 sequesters preformed beta-gal in the periplasm, suggesting leakage of beta-gal as the means by which this assay detects compound activities. A model is proposed according to which beta-gal release by BAU-102 reflects activation of pathways leading to autolysis. The authors also report a simplified high-throughput assay using BAU-102 combined with the fluorogenic substrate N-methylumbelliferyl-beta-D-galactoside as a single reagent. Cell wall inhibitors release beta-gal consistently only after 60 min of incubation, whereas compounds with surfactant activity show an almost immediate release. A high-throughput screen of a 480-compound library of known bioactives yielded 8 compounds that cause beta-gal release. These results validate the BAU-102 assay as an effective tool in antimicrobial drug discovery.

Validation for high-throughput screening of a VanRS-based reporter gene assay for bacterial cell wall inhibitors

AIMS

The present study was undertaken to validate, for antibiotic discovery, a reporter gene assay based on a Bacillus subtilis strain expressing the Enterococcusfaecium vanRS genes and a vanH-lacZ fusion, which produced beta-galactosidase activity in the presence of cell wall inhibitors (CWI) and lysozyme.

METHODS AND RESULTS

The reporter assay was miniaturized, automated and validated with antibiotics and tested against portions of chemical and microbial extract libraries. The assay is simple, fast and reproducible and can detect all CWI, sometimes at concentrations lower than those necessary to inhibit bacterial growth. However, some membrane-interfering compounds also generate comparable signals. While most CWI elicit a signal that is transcription-dependent and abolished in an osmoprotective medium, transcription is not required for beta-galactosidase activity brought about by the membrane-interfering compounds.

CONCLUSIONS

At least two distinct mechanisms appear to lead to enzymatic activity in the reporter strain. Effective counterscreens can be designed to discard the undesired classes of compounds.

SIGNIFICANCE AND IMPACT OF THE STUDY

Extensive validation is required before introducing a reporter assay in high-throughput screening. However, the ease of operation and manipulation makes the reporter assays powerful tools for antibiotic discovery.

Lactobacillus farciminis MD, a newer strain with potential for bacteriocin and antibiotic assay

A native isolate Lactobacillus farciminis MD isolated from fermenting mushroom exhibited a high degree of sensitivity to the majority of the bacteriocins produced by strains of lactobacilli, leuconostoc and pediococci. Also, the efficacy of Lact. farciminis MD as a sensitive strain for antibiotic assay was established against different antibiotics including ampicillin, cefazoline, chloramphenicol and nitrofurantoin at concentrations of 30 microg each, showing an inhibition zone of 30 mm diameter. The high degree of sensitivity towards bacteriocins and antibiotics provide potential for the exploitation of Lact. farciminis MD in establishing very well-defined bacteriocin producers.