In vitro miconazole susceptibility of meticillin-resistant Staphylococcus pseudintermedius and Staphylococcus aureus

Background – The emergence and dissemination of meticillin‐resistant staphylococci has created significant treatment challenges in veterinary medicine and increased interest in topical therapy for superficial infections. Concern has been expressed regarding the use of some topical antimicrobials in animals because of the potential for emergence of resistance, and additional options are required. Miconazole has limited antibacterial properties that include antistaphylococcal activity. Hypothesis/Objectives – The objective of this study was to assess the in vitro susceptibility of Staphylococcus pseudintermedius and Staphylococcus aureus to miconazole. Methods – In vitro susceptibility of 112 meticillin‐resistant S. pseudintermedius (MRSP), 53 meticillin‐resistant S. aureus (MRSA) and 37 meticillin‐susceptible S. pseudintermedius (MSSP) to miconazole was assessed using agar dilution. Results – The minimal inhibitory concentration (MIC) range, MIC₅₀ and MIC₉₀ for MRSP were 1–8, 2 and 4 μg/mL, respectively. Corresponding results for MRSA were 1–8, 2 and 6 μg/mL, and for MSSP 1–4, 2 and 2 μg/mL. The MIC for MSSP was a significantly lower MIC than that for both MRSP (P = 0.006) and MRSA (P < 0.001), while the MIC for MRSP was significantly lower than that for MRSA (P = 0.001). Conclusions and clinical importance – These in vitro data suggest that miconazole could be a useful therapeutic option for superficial infections caused by meticillin‐susceptible and meticillin‐resistant staphylococci, but proper clinical investigation is required.     

In vitro antimicrobial activity of orbifloxacin against Staphylococcus intermedius isolates from canine skin and ear infections

The objective of the study was to evaluate the in vitro activity of orbifloxacin against Staphylococcus intermedius strains isolated in France from canine skin and ear infections. The minimum inhibitory concentrations (MICs) of orbifloxacin against 240 field S. intermedius isolates (69 skin and 171 ear isolates) ranged from 0.016 to 8 mg l(-1), with MIC50 and MIC90 equal to 0.5 and 1 mg l(-1), respectively. Only one strain, a pyoderma isolate was resistant (MIC=8 mg l(-1)). Orbifloxacin was tested at different concentrations for killing rate against five isolates obtained from pyoderma cases and against a reference strain (Staphylococcus aureus ATCC 29213). Orbifloxacin expressed a concentration-dependent bactericidal activity against the S. aureus reference strain, but a time-dependent bactericidal activity against S. intermedius. Orbifloxacin induced bactericidal effect against the S. intermedius strains tested with concentrations equal to or two times MIC.     

In vitro antimicrobial activity of nitrofurantoin against Escherichia coli and Staphylococcus pseudintermedius isolated from dogs and cats

Minimum inhibitory concentrations (MIC) of nitrofurantoin were determined by agar dilution in 269 canine and feline isolates of Escherichia coli and Staphylococcus pseudintermedius, two of the most common bacterial species associated with urinary tract infection (UTI) in small animals. The MIC90 for E. coli and S. pseudintermedius were 32 and 16 μg/ml, respectively. All isolates, including multidrug-resistant strains of known genetic background, displayed MICs below the drug concentrations reported in canine urine following oral administration of nitrofurantoin. Preliminary data on mutant prevention concentration (MPC) and many years of nitrofurantoin usage in human medicine suggest that emergence of resistant mutants during treatment is not a critical issue for this drug. The study provides species-specific data on nitrofurantoin MIC distribution that can be used for setting dog- and cat-specific breakpoints. Although nitrofurantoin is not an appropriate first-line agent for empirical treatment of canine UTI due to toxicity and poor pharmacokinetic properties, it may be indicated for treatment of UTI caused by multidrug-resistant bacteria, which are otherwise difficult to treat using conventional veterinary antimicrobial agents.     

In vitro antimicrobial efficacy of β‐defensin 3 against Staphylococcus pseudintermedius isolates from healthy and atopic canine skin

β‐Defensins (BDs) are highly conserved antimicrobial peptides important in innate defence against bacteria. β‐Defensin 3 has a specific role in protecting the skin. This study quantified the minimal inhibitory concentration (MIC) of human (h)BD3 against Staphylococcus pseudintermedius isolates from atopic and healthy dogs. Single colony isolates (1 × 10⁵ colony‐forming units/mL log phase) were cultured with doubling dilutions of hBD3 in sodium phosphate buffer from 0.8 to 50 μg/mL at 37 °C for 2 h, before adding 100 μL of tryptone soy broth and incubating for a further 20 h. Bacterial growth was assessed as the mean optical density at 540 nm corrected for background. The median MIC was 12.5 μg hBD3/mL (range 3.125–25 μg/mL; n = 22). Forty‐five percent of the isolates were inhibited at ≤6.25 μg hBD3/mL, and 90% were inhibited at ≤12.5 μg hBD3/mL. Bacterial growth was not inhibited at ≤1.6 μg hBD3/mL. There were no significant differences in the inhibition by hBD3 of isolates from atopic (median MIC 12.5 μg/mL, range 6.25–25 μg/mL, n = 14) and healthy dogs (median MIC 9.4 μg/mL, range 3.125–12.5 μg/mL, n = 8); from noninfected colonized sites (median MIC 12.5 μg/mL, range 3.125–25 μg/mL, n = 16) and infected lesions (median MIC 9.4 μg/mL, range 6.25–12.5 μg/mL, n = 6); or between sample sites (nose median MIC 12.5 μg/mL, range 6.25–25 μg/mL, n = 5; perineum median MIC 12.5 μg/mL, range 3.125–25 μg/mL, n = 7; ear median MIC 6.25 μg/mL, range 6.25–12.5 μg/mL, n = 4; lesions median MIC 9.4 μg/mL, range 6.25–12.5 μg/mL, n = 6). In conclusion, hBD3 inhibited the growth of canine S. pseudintermedius isolates in vitro irrespective of origin.     

Antimicrobial activity of gallium maltolate against Staphylococcus aureus and methicillin-resistant S. aureus and Staphylococcus pseudintermedius: An in vitro study

Gallium is a trivalent semi-metallic element that has shown antimicrobial activity against several important human pathogens. This antimicrobial activity is likely related to its substitution in important iron-dependent pathways of bacteria. The genus Staphylococcus, which includes human and animal pathogens that cause significant morbidity and mortality, requires iron for growth and colonization. In this study, gallium maltolate, at various concentrations between 50 and 200μM, inhibited the in vitro growth of Staphylococcus aureus and methicillin-resistant S. aureus (MRSA) at time-points between 8 and 36h after inoculation. The inhibitory activity of gallium maltolate against clinical isolates of MRSA and methicillin-resistant Staphylococcus pseudintermedius (MRSP) from a veterinary teaching hospital was determined.     

In vitro activity of difloxacin against canine bacterial isolates.

The in vitro activity of difloxacin against canine bacterial isolates from clinical cases was studied in the United States and The Netherlands. Minimal inhibitory concentrations (MIC), the postantibiotic effect, the effect of pH on antimicrobial activity, and the bacterial killing rate tests were determined according to standard techniques. The MICs of American and Dutch isolates agreed in general. The MICs of the American gram-negative isolates ranged from 0.06 to 2.0 microg/ml, and the MICs of the Dutch gram-negative isolates ranged from 0.016 to 8.0 microg/ml. A few European strains of Proteus mirabilis and Klebsiella pneumoniae had relatively high MICs. Bordetella bronchiseptica also was less susceptible to difloxacin. The MICs of the American gram-positive cocci ranged from 0.125 to 4.0 microg/ml, and the MICs of Dutch isolates ranged from 0.125 to 2.0 microg/ ml. Difloxacin induced a concentration-dependent postantibiotic effect that lasted 0.2-3 hours in cultures with Escherichia coli, Staphylococcus intermedius, Streptococcus canis, Proteus spp., and Klebsiella pneumoniae. There was no postantibiotic effect observed against canine Pseudomonas aeruginosa. Decreasing the pH of the medium increased the MIC of Proteus mirabilis for difloxacin. The MICs of Escherichia coli and Klebsiella pneumoniae were lowest at neutral pH and were slightly increased in acid or alkaline media. At a neutral pH, most tested bacterial species were killed at a difloxacin concentration of 4 times the MIC. Similar results were obtained when these same bacteria were tested against enrofloxacin. A Klebsiella pneumoniae strain in an acidic environment was readily killed at difloxacin or enrofloxacin MIC, but at neutral pH the drug concentration had to be raised to 4 times the MIC for a bactericidal effect. After 24 hours of incubation at pH 7.1, difloxacin and enrofloxacin had similar bactericidal activity for all bacteria tested except Staphylococcus intermedius. Against S. intermedius, difloxacin was more bactericidal than enrofloxacin.     

In vitro evaluation of topical biocide and antimicrobial susceptibility of Staphylococcus pseudintermedius from dogs

Background – Staphylococcus pseudintermedius is an important canine pathogen, and the emergence and widespread dissemination of meticillin‐resistant strains (MRSP) is of significant concern. Multidrug‐resistant infections may require alternative approaches, such as the use of topical therapy. There is minimal information about the in vitro susceptibility of meticillin‐susceptible S. pseudintermedius (MSSP) and MRSP to biocides and topical antimicrobials. Hypothesis/Objectives – The hypothesis was that clinical isolates of MSSP and MRSP would not have universal susceptibility to topical biocides and antimicrobials. The goal of this study was to assess the susceptibility of a collection of S. pseudintermedius isolates to selected antimicrobials and biocides. Animals – The study was performed on clinical isolates of MSSP and MRSP from dogs with skin and soft tissue infections collected throughout North America between 2006 and 2008. Methods – The minimal inhibitory concentrations (MICs) of chlorhexidine digluconate, benzalkonium chloride, triclosan, accelerated hydrogen peroxide, geranium oil, tea tree oil and grapefruit seed extract were tested for 25 MRSP and 25 MSSP isolates from dogs using the agar dilution method. The MICs of fusidic acid, bacitracin and mupirocin were determined using Etests. Results – Triclosan demonstrated excellent activity against all bacterial isolates, with no growth at the lowest concentration evaluated (MIC ≤ 0.5 μg/mL). Conversely, grapefruit seed extract did not inhibit growth at the highest concentration tested (MIC > 3.84 μg/mL). All isolates were susceptible to mupirocin, fusidic acid and bacitracin. There were no significant differences noted in the range, MIC₅₀ or MIC₉₀ between MSSP and MRSP isolates. Conclusions and clinical importance – While isolates were susceptible to most of the tested compounds, universal susceptibility to all compounds with potential antimicrobial activity cannot be assumed, and specific testing is required.     

In vitro activity of fifteen antimicrobial agents against methicillin-resistant and methicillin-susceptible Staphylococcus intermedius

In this study the susceptibility of 91 methicillin-resistant and -susceptible Staphylococcus intermedius strains (MRSI and MSSI, respectively) against 15 antimicrobial agents was determined. The activity of the antimicrobial agents was studied at pH 7.2 and pH 8.5. Methicillin was more active at pH7.2 (28 strains methicillin-resistant) than at pH 8.5 (55 strains methicillin-resistant). Gentamicin showed excellent activity, with only 3 strains resistant at pH 8.5. However, gentamicin would have to be administered parenterally. Oxytetracy-cline cannot be recommended for treatment of canine staphylococcal dermatitis, due to the high percentage (over 25%) of strains that were found to be resistant. Clindamycin showed little activity in inhibiting growth of the strains studied, the percent resistance at pH 7.2 was 93.4%. Rifampin behaved differently at the two pH values. However, a close relationship was noted between methicillin-resistant and rifampin-resistant strains, particularly at the lower pH. Of the fluoroquinolones, ciprofloxacin or enrofloxacin would be a good useful alternative for the treatment of methicillin-resistant strains of S. intermedius . Lastly, very high resistance to sulphamethoxypyridazine was found, as was the case with trimethoprim and a combination of trimethoprim/ sulphamethoxypyridazine, against not only MRSI but also MSSI strains.     

In vitro antiseptic susceptibilities for Staphylococcus pseudintermedius isolated from canine superficial pyoderma in Japan

Background –  Topical therapy, particularly with chlorhexidine, is becoming increasingly common as a treatment option for canine pyoderma; however, there are limited studies on the susceptibility of Staphylococcus pseudintermedius to chlorhexidine compounds. Objectives –  To determine the in vitro susceptibility of both meticillin‐resistant and meticillin‐susceptible S. pseudintermedius isolates to chlorhexidine and other antiseptic agents and the presence of multidrug efflux pump genes. Samples –  One hundred S. pseudintermedius isolates from 23 initial and 77 recurrent cases of canine pyoderma. Methods –  After bacterial identification and mecA testing, minimal inhibitory concentrations (MICs) of antiseptic agents were determined. Multidrug efflux pump genes, including qacA, qacB and smr, were identified. Results –  Of the 100 isolates, 57 were identified as meticillin‐resistant S. pseudintermedius. The MIC₉₀ of chlorhexidine acetate, chlorhexidine gluconate, acriflavine, ethidium bromide and benzalkonium chloride were 1, 1, 2, 0.5 and 2 μg/mL, respectively. Multidrug efflux pump genes qacA, qacB and smr were not detected in any of the isolates. Conclusions and clinical importance –  The MICs for chlorhexidine and other antiseptics remain low, and multidrug efflux pump genes were not found in the tested isolates.