Post-antibiotic effect of orbifloxacin against Escherichia coli and Pseudomonas aeruginosa isolates from dogs

Post-Antibiotic and Post-Antibiotic Sub-Minimum Inhibitory Concentration Effects of Carvacrol against Salmonella Typhimurium

Carvacrol is a compound present in essential oils with proven antimicrobial activity against numerous pathogens. We firstly determine the post-antibiotic effect (PAE) of carvacrol (1×, 2×, 4× MIC) and post-antibiotic sub-minimum inhibitory concentration (MIC) effect (1× + 0.25× MIC and 2× + 0.25× MIC) for two concentrations of Salmonella Typhimurium ATCC14028 (106 and 108 CFU/mL). Prior to testing, the minimum concentration and exposure time to achieve the bacterial inhibition (MIC 0.6 mg/mL and 10 min) were determined by broth microdilution and time-kill curve methods, respectively. At the MIC, carvacrol did not generate any PAE. At twice the MIC, the PAE was 2 h with the standard inoculum (106 CFU/mL) and 1 h with the high-density inoculum (108 CFU/mL). At 4× MIC concentrations, the PAE was higher in both cases > 43.5 h. Continuous exposure of post-antibiotic phase bacteria (1× and 2× MIC) to carvacrol at 0.25× MIC (0.15 mg/mL) resulted in an increase in PAE (PA-SME) above 43.5 h with both inocula. These results suggest that the PA-SME of carvacrol for S. Typhimurium can be significantly prolonged by increasing the sub-MICs, which would allow dose spacing, reduce adverse effects and improve its efficacy in the treatment of infected animals and as a disinfectant in agri-food facilities.

Ex vivo pharmacokinetic/pharmacodynamic relationship of valnemulin against Clostridium perfringens in plasma, the small intestinal and caecal contents of rabbits

The pharmacokinetic (PK) and ex vivo pharmacodynamic (PD) of valnemulin against Clostridium perfringens were investigated in plasma, the small intestinal and caecal contents of rabbits following intravenous (IV) or oral administration at 3 mg/kg bodyweight (BW). The postantibiotic effect (PAE) and postantibiotic sub-MIC effect (PA-SME) of valnemulin against C. perfringens ATCC13124 were also determined. The time-kill curves were established in vitro and ex vivo to evaluate the antibacterial activity of valnemulin against C. perfringens. The elimination half-lives (T1/2λz) of valnemulin in the jejunal fluids (7.82 h) or caecal contents (14.8 h) of rabbits was significantly longer than that in plasma (2.94 h). The MIC values of valnemulin against C. perfringens ATCC13124 were both 0.063 μg/mL in the artificial medium and jejunal fluids. The PAEs of valnemulin against C. perfringens were 2.9 h (1 × MIC) and 5.03 h (4 × MIC), and the PA-SMEs ranged from 7.9 h to 11.1 h. Valnemulin exhibited rapid, time-dependent killing feature, and the ex vivo dose-response profile was closely fitted to sigmoid Emax model (r(2) = 0.9985). The surrogate index of AUC24 h/MIC ratios required to achieve the bactericidal and virtual bacterial elimination effects were 57.5 and 90.1 h, respectively. Accordingly, the calculated daily dosage regimens of valnemulin for the bactericidal activity (1.96 mg/kg) and bacterial elimination (3.08 mg/kg) would be therapeutically effective in rabbits against C. perfringens with MIC ≤0.5 μg/mL.

Effects of intracanal irrigant MTAD Combined with nisin at sub-minimum inhibitory concentration levels on Enterococcus faecalis growth and the expression of pathogenic genes

Exposure to antibiotics is considered to be the major driver in the selection of antibiotic-resistant bacteria and may induce diverse biological responses in bacteria. MTAD is a common intracanal irrigant, but its bactericidal activity remains to be improved. Previous studies have indicated that the antimicrobial peptide nisin can significantly improve the bactericidal activity of MTAD against Enterococcus faecalis. However, the effects of MTAD and its modification at sub-minimum inhibitory concentration (sub-MIC) levels on Enterococcus faecalis growth and the expression of pathogenic genes still need to be explored. In this study, the results of post-antibiotic effects (PAE) and post-antibiotic sub-MIC effects (PASME) showed that MTADN (nisin in combination with MTAD) had the best post-antibiotic effect. E. faecalis after challenge with MTAD was less sensitive to alkaline solutions compared with MTAN (nisin in place of doxycycline in MTAD) and MTADN. E. faecalis induced with sub-MIC of MTAD generated resistance to the higher concentration, but induction of E. faecalis with MTAN did not cause resistance to higher concentrations. Furthermore, real-time polymerase chain reaction (RT-PCR) showed that the stress caused by sub-MIC exposure to MTAD, MTAN, or MTADN resulted in up- or down-regulation of nine stress genes and four virulence-associated genes in E. faecalis and resulted in different stress states. These findings suggested that nisin improved the post-antibacterial effect of MTAD at sub-MIC levels and has considerable potential for use as a modification of MTAD.

Postantibiotic effect and postantibiotic sub-minimum inhibitory concentration effect of valnemulin against Staphylococcus aureus isolates from swine and chickens

The postantibiotic effect (PAE) and postantibiotic sub-minimum inhibitory concentration (MIC) effect (PA-SME) of valnemulin against Staphylococcus aureus were investigated in vitro using a spectrophotometric technique and classic viable count method. A standard curve was constructed by regression analysis of the number of colonies and the corresponding optical density (OD) at 630 nm of the inoculum. After exposure to valnemulin at different concentrations for an hour, the antibiotic was removed by centrifuging and washing. The PA-SMEs were measured after initial exposure to valnemulin at 4 × the MIC, and then, valnemulin was added to reach corresponding desired concentrations in the resuspended culture. Samples were collected hourly until the culture became turbid. The results were calculated by converting the OD values into the counts of bacteria in accordance with the curve. The MIC of valnemulin against eight strains was identically 0.125 μg ml(-1) . The mean PAEs were 2.12 h (1 × MIC) and 5.06 h (4 × MIC), and the mean PA-SMEs were 6.85 h (0.1 × MIC), 9.12 h (0.2 × MIC) and 10.8 h (0.3 × MIC). The results showed that the strains with identical MICs exhibited different PAEs and PA-SMEs. Valnemulin produced prolonged PAE and PA-SME periods for Staph. aureus, supporting a longer dosing interval while formulating a daily administration dosage.

SIGNIFICANCE AND IMPACT OF THE STUDY

In this study, valnemulin demonstrated prolonged postantibiotic effects and postantibiotic sub-MIC effects on strains of Staphylococcus aureus. The strains with identical MICs of valnemulin exhibited different PAEs and PA-SMEs. Staphylococcus aureus isolated from different species has little impact on the postantibiotic effect of valnemulin. The result suggests a longer dosing interval while formulating a daily administration dosage, and it may play a valuable role of valnemulin in treating Staph. aureus infections in animals.