PSXIV-15 Synergistic Inhibition of Staphylococcal Growth by Select Nitrocompounds and Sodium Chlorate

Staphylococci are important mastitic-pathogens of humans and livestock. Chlorate is reported to be a potent inhibitor against staphylococci but adaptation by the microbes can occur. Presently, we cultured multi-drug resistant Staphylococcus aureus strain ATCC 12600 anaerobically at 39oC in nitrate-supplemented (5 mM) Tryptic Soy broth. The broth was treated without (control) or alone with 5 mM sodium chlorate (ClO3) or 9 mM of either 2-nitro-1-propanol (NPOH), ethyl nitroacetate (ENA) or ethyl 2-nitropropionate (E2NPA) or with combinations of CLO3 and respective nitrocompound. Control and treated cultures were incubated in triplicate and growth was measured via absorbance (600 nm) at during 9 h. An analysis of variance revealed an inhibitory effect of treatments (P < 0.05; SEM = 0.078) on mean specific growth rates of S. aureus, with rates during logarithmic growth being most rapid in controls and cultures treated with ENA or EN2PA (1.45, 1.23 and 1.21 h-1, respectively) and more than 92% slower in NPOH and NPOH+CLO3-treated cultures (0.09 and 0.03 h-1, respectively). Growth rates of cultures treated solely with CLO3 (0.66 h-1) were more than 50% slower than controls and were decreased further in cultures treated with combinations of CLO3 plus E2NPA or ENA (0.39 and 0.45 h-1). After 9 h of growth, maximum optical densities (maxOD) were highest for controls and cultures treated with ENA or EN2PA (0.52, 0.52 and 0.50 maxOD, respectively), intermediate in cultures treated with CLO3 alone (0.21 maxOD) and lower yet in cultures treated with CLO3 plus ENA, E2NPA or NPOH (0.15, 0.11 and 0.02 maxOD, respectively). The maxOD for cultures treated with NPOH alone was 0.02 and did not differ from cultures treated with the combination of CLO3+NPOH. Results demonstrate marked synergistic and prolonging effects of combing ENA and E2NPA with CLO3 but no additional effect of the potent anti-staphylococcal activity of NPOH.