Effects of Fluoroquinolone Exposure to Growth and Morphology in Staphylococcus aureus

Under exposure to fluoroquinolone (FQ) stressors, Staphylococcus aureus generates resistance through adaptive responses via alterations of target enzymes, efflux pumps, morphology, or general gene expression and mutation. This study examined the effects of continuous FQs exposure on the growth and morphology of bacterial colonies and cells. S. aureus ATCC 29213 and six FQ-exposed strains, which were induced through in vitro exposing S. aureus ATCC 29213 to sub-MIC value of ciprofloxacin (CIP), ofloxacin (OFL), and levofloxacin (LEV) were included in the study. Growth of these seven S. aureus strains in Luria-Bertani broth (LB), Mueller-Hinton broth (MHB), and Nutrient broth (NB) was investigated using plate counting method while light microscope and scanning electron microscope were used to identify changes in colony and cell morphology. Regarding growth analysis, most FQ-exposed S. aureus showed significantly lower growth rates in LB and MHB but higher in NB compared to the initial strain. Besides, nearly all FQ-exposed S. aureus experienced a decrease by about 28% in normal-colony perimeter and attained a roughly 22% increase in single-cell size compared to the initial strain. The percentage of small colony variants (SCV) of most FQ-exposed strains increased compared to the initial S. aureus. This study suggested that the adaptive response mechanisms to FQ exposure are associated with changes in the bacterial metabolism which resulted in a decrease of growth in nutrient-rich medium but an increase in nutrient-less one. Besides, S. aureus decreased its normal colony perimeter but increased the SCV and single-cell size percentage under serial exposure to FQs.