‘Resistance Decay’ Effect in a Strain of Staphylococcus arlettae (BMS7), Exposed to Linezolid through a ‘Serial Passage’ Study
Rishika Panda
School of Life Sciences, Sambalpur University, Jyoti Vihar, Burla, Sambalpur, Odisha-768019, India.
Smaranika Pattnaik *
Laboratory of Medical Microbiology, School of Life Sciences, Sambalpur University, Jyoti Vihar, Burla, Sambalpur, Odisha-768019, India.
*Author to whom correspondence should be addressed.
Abstract
Background: Staphylococcus arlettae is an opportunistic coagulase-negative bacterium associated with antimicrobial resistance. However, changes in its susceptibility to linezolid following repeated antibiotic exposure through serial passage remain poorly understood.
Aim: This study evaluated the apparent linezolid-susceptibility phenotype of Staphylococcus arlettae BMS7 during serial passage.
Methods: BMS7 was isolated from air-exposed nutrient agar plates placed near a tertiary-care hospital in Burla. Colony observations, Gram staining, 16S rRNA sequencing, BLASTn analysis and phylogenetic comparison were used for characterisation. Initial linezolid susceptibility was assessed using agar-cup diffusion, tube dilution and pour-plate methods. A modified replica-plating procedure compared growth on mannitol agar plates containing 120 µg/ml linezolid with growth on antibiotic-free control plates during successive passages. Post-passage assessments included agar-cup diffusion, pour-plate testing and scanning electron microscopy.
Results: Before passage, BMS7 grew at the tested linezolid concentrations, including 120 µg/ml. During serial passage, some inoculated grids on linezolid-containing plates showed absent or reduced colony growth compared with control plates. Following passage, inhibition zones were observed in agar-cup assays, while the pour-plate assay showed no growth at the reported concentrations ranging from 10 to 120 µg/ml. Electron microscopy documented altered cellular appearances following linezolid exposure.
Conclusion: The observations indicate a change in the apparent linezolid-susceptibility phenotype of BMS7 under the reported experimental conditions. They do not establish the mechanism, stability or clinical relevance of this change.
Keywords: Staphylococcus arlettae, BMS7, linezolid, serial passage, replica plating, antimicrobial resistance, susceptibility phenotype, tube dilution, agar-cup diffusion, scanning electron microscopy