Comparative minimum inhibitory and mutant prevention drug concentrations of enrofloxacin, ceftiofur, florfenicol, tilmicosin and tulathromycin against bovine clinical isolates of Mannheimia haemolytica.

Mannheimia haemolytica is the most prevalent cause of bovine respiratory disease (BRD) and this disease accounts for 75% of morbidity, 50-70% of feedlot deaths and is estimated to cost up to $1 billion dollars annually in the USA. Antimicrobial therapy is essential for reducing morbidity, mortality and impacting on the financial burden of this disease. Due to the concern of increasing antimicrobial resistance, investigation of antibacterial agents for their potential for selecting for resistance is of paramount importance. A novel in vitro measurement called the mutant prevention concentration (MPC) defines the antimicrobial drug concentration necessary to block the growth of the least susceptible cells present in high density (≥10(7) colony forming units/ml) bacterial populations such as those seen in acute infection. We compared the minimum inhibitory concentration (MIC) and MPC values for 5 antimicrobial agents (ceftiofur, enrofloxacin, florfenicol, tilmicosin, tulathromycin) against 285 M. haemolytica clinical isolates. The MIC(90)/MPC(90) values for each agent respectively were as follows: 0.016/2, 0.125/1, 2/≥16, 8/≥32, 2/8. Dosing to achieve MPC concentrations (where possible) may serve to reduce the selection of bacterial subpopulations with reduced antimicrobial susceptibility. The rank order of potency based on MIC(90) values was ceftiofur > enrofloxacin > florfenicol = tulathromycin > tilmicosin. The rank order of potency based on MPC(90) values was enrofloxacin > ceftiofur > tulathromycin > florfenicol ≥ tilmicosin.

In vitro killing of Escherichia coli, Staphylococcus pseudintermedius and Pseudomonas aeruginosa by enrofloxacin in combination with its active metabolite ciprofloxacin using clinically relevant drug concentrations in the dog and cat.

Enrofloxacin is a fluoroquinolone antibacterial agent used to treat infections in companion animals. Enrofloxacin's antimicrobial spectrum includes Gram positive and Gram-negative bacteria and demonstrates concentration-dependent bacteriocidal activity. In dogs and cats, enrofloxacin is partially metabolized to ciprofloxacin and both active agents circulate simultaneously in treated animals at ratios of approximately 60-70% enrofloxacin to 30-40% ciprofloxacin. We were interested in determining the killing of companion animal isolates of Escherichia coli, Staphylococcus pseudintermedius and Pseudomonas aeruginosa by enrofloxacin and ciprofloxacin combined using clinically relevant drug concentrations and ratios. For E. coli isolates exposed to 2.1 and 4.1µg/ml of enrofloxacin/ciprofloxacin at 50:50, 60:40 and 70:30 ratios, a 1.7-2.5log(10) reduction (94-99% kill) was seen following 20min of drug exposure; 0.89-1.7log(10) (92-99% kill) of S. pseudintermedius following 180min of drug exposure; 0.85-3.4log(10) (98-99% kill) of P. aeruginosa following 15min of drug exposure. Killing of S. pseudintermedius was enhanced in the presence of enrofloxacin whereas killing of P. aeruginosa was enhanced in the presence of ciprofloxacin. Antagonism was not seen when enrofloxacin and ciprofloxacin were used in kill assays. The unique feature of partial metabolism of enrofloxacin to ciprofloxacin expands the spectrum of enhanced killing of common companion animal pathogens.

Antimicrobial efficacy of gatifloxacin and moxifloxacin with and without benzalkonium chloride compared with ciprofloxacin and levofloxacin against methicillin-resistant Staphylococcus aureus.

We compared the antimicrobial activity of gatifloxacin and moxifloxacin with and without benzalkonium chloride (BAK) against clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA). Minimum inhibitory concentrations (MICs) against clinical isolates of MRSA were evaluated. Approximately 10(5 )CFU/ml of methicillinresistant S. aureus was added to Mueller-Hinton broth containing two-fold concentration increments of drug. For the evaluation of gatifloxacin with BAK, 50 microg/ml of BAK were added to the first well of the plate with gatifloxacin or moxifloxacin and then serially diluted. The combination of gatifloxacin or moxifloxacin with BAK was more active than either fluoroquinolone without BAK. The MICs ranged from

Application of two methods to determine killing of Streptococcus pneumoniae by various fluoroquinolones.

Minimum inhibitory concentration (MIC) testing measures the lowest drug concentration that prevents microbial growth using an inoculum of 10(5) colony forming units/ml (cfu/ml) whereas the mutant prevention concentration (MPC) (inoculum approximately 10(10) cells) defines the antimicrobial drug concentration threshold that would require an organism to possess two simultaneous mutations for continued growth in the presence of the drug. The rates at which multidrug-resistant Streptococcus pneumoniae [MDRSP] were killed by the respiratory fluoroquinolones, gatifloxacin, gemfloxacin, levofloxacin and moxifloxacin, were compared based on the MIC and MPC drug concentrations and at inocula ranging from 10(6)-10(9) cfu/ml. The MIC drug concentration failed to eradicate all viable cells whereas the MPC drug concentration resulted in 99.9% to 100% cellular reduction following 12-24 hours of drug exposure. MPC values against S. pneumoniae were different for each fluoroquinolone. The MPC drug concentration prevents the selection of multidrug-resistant or fluoroquinolone-resistant S. pneumoniae. The value of dosing of antimicrobial agents based on MPC thresholds results in a rapid reduction in viable cells--even at higher inocula which are more reflective of organism burden in pneumonia. The rapid reduction in viable cells observed at MPC drug concentrations may not only have an impact on preventing the selection of resistant mutants but may also help explain the rapid symptom resolution seen with new fluoroquinolones since these agents lead to little or low release of cell contents which are known to drive the inflammatory response.

In vitro susceptibility of 4903 bacterial isolates to gemifloxacin--an advanced fluoroquinolone.

The in vitro activity of gemifloxacin against over 4900 bacterial isolates was determined by microbroth dilution with interpretation in accordance with NCCLS guidelines. Susceptibility results were compared with those for ciprofloxacin, gatifloxacin, levofloxacin and moxifloxacin. Gemifloxacin and the other fluoroquinolones were not affected by either beta-lactamase production or penicillin-resistance in Streptococcus pneumoniae. The MIC90 values for gemifloxacin were: S. pneumoniae 0.063 mg/l; Haemophilus influenzae 0.016 mg/l; Moraxella catarrhalis 0.008 mg/l, methicillin-susceptible Staphylococcus aureus 0.063 mg/l; methicillin-susceptible Streptococcus pyogenes 0.031 mg/l; Enterobacteriaceae 0.031-0.16 mg/l; Pseudomonas aeruginosa 4 mg/l; Neisseria meningitidis 0.008 mg/l. The MIC90 for gemifloxacin was lower than those for the other quinolones tested against S. pneumoniae (ciprofloxacin 2-4 mg/l, gatifloxacin 0.5 mg/l, levofloxacin 1-2 mg/l, moxifloxacin 0.25 mg/l). This study confirms the enhanced potent activity of gemifloxacin against Gram-positive pathogens, its broad-spectrum, Gram-negative activity and indicates that gemifloxacin is likely to have an important role in treating patients with Gram-positive and/or Gram-negative infections.

Susceptibility of Canadian isolates of Haemophilus influenzae, Moraxella catarrhalis and Streptococcus pneumoniae to oral antimicrobial agents.

We measured the susceptibility of Canadian isolates of three respiratory tract pathogens (Haemophilus influenzae, Moraxella catarrhalis and Streptococcus pneumoniae) to several currently approved antimicrobial agents by two different methods. We also measured the susceptibility of isolates to seven fluoroquinolones. Beta-lactamase was produced by 123/566 (21.7%) of H. influenzae isolates compared with 178/200 (89%) of M. catarrhalis isolates. For S. pneumoniae 83/374 (22.2%) isolates were penicillin resistant and of these 2.1% (8/374) showed high level resistance (MIC > or = 2 mg/l). Regardless of methodology, all fluoroquinolones were highly active against H. influenzae (MIC(90) < or = 0.031 mg/l) and M. catarrhalis (MIC(90) < or = 0.064 mg/l) isolates. Susceptibility of H. influenzae to cefuroxime and amoxycillin/clavulanic acid was 99-100% whereas 84-85.5% were susceptible to cefaclor and cefprozil. Azithromycin susceptibility ranged from 82.6 to 99.2% depending on the method. M. catarrhalis isolates were uniformly susceptible to all agents tested except amoxycillin. Cross-resistance in S. pneumoniae to all non-quinolone agents was concurrent with increasing penicillin resistance as shown by increasing MIC90 values. For the fluoroquinolones tested, the rank order of potency based on MIC(90) values was as follows: gemifloxacin (0.031-0.063 mg/l), trovafloxacin (0.125 mg/l), moxifloxacin (0.125-0.25 mg/l), grepafloxacin (0.125-0.25 mg/l), gatifloxacin (0.5 mg/l), levofloxacin (1 mg/l) and ciprofloxacin (2 mg/l). Our study confirms either a high or increasing prevalence of antimicrobial resistant respiratory pathogens in Canada and also compares the new and old fluoroquinolones and their potential role as therapy for community-acquired infections. The prevalence of beta-lactamase positive H. influenzae may have decreased from levels reported in previous studies.

In-vitro activity of cefepime and seven other antimicrobial agents against 1518 non-fermentative Gram-negative bacilli collected from 48 Canadian health care facilities. Canadian Afermenter Study Group.

Non-fermentative bacilli are primarily nosocomial pathogens, and are also often resistant in vitro to a broad range of antimicrobial agents. In this large Canadian study, we collected 1466 clinical, non-repeat isolates of Pseudomonas aeruginosa, 21 of Acinetobacter spp. and 31 Stenotrophomas maltophilia. MICs of eight antibiotics were determined by the NCCLS microdilution method in a central laboratory. Tobramycin was the most active agent against P. aeruginosa (94.5% susceptible); amikacin and imipenem were the most active against Acetinobacter spp. (100%) and ceftazidime was the most active against S. maltophilia (40.6%). Against each group of isolates, cefepime was active against 87, 86.4 and 15.6%, respectively. This in-vitro study showed that cefepime may be a useful additional agent in the treatment of infections caused by P. aeruginosa and Acinetobacter spp., but not when S. maltophilia is considered pathogenic.

Determination of the antimicrobial susceptibilities of Canadian isolates of Haemophilus influenzae, Streptococcus pneumoniae and Moraxella catarrhalis. Canadian Antimicrobial Study Group.

The susceptibility of Canadian isolates of three respiratory tract pathogens (Haemophilus influenzae, Moraxella catarrhalis and Streptococcus pneumoniae) to several antimicrobial agents were tested by two different methods. Beta-lactamase was produced by 68/211 (32.2%) of H. influenzae isolates and 64/75 (85.3%) of M. catarrhalis isolates. For S. pneumoniae, 19/156 (12.2%) isolates were resistant to penicillin (MIC > or = 0.12 mg/L) and two isolates had MICs of 1.5 mg/L. For some combinations of agents and organisms, different methods gave different values for the proportion of isolates susceptible. Regardless of methodology, for H. influenzae, the most active antimicrobials based on proportion of strains susceptible were ciprofloxacin (100%) and cefpodoxime (98.5-100%). For M. catarrhalis, the most active agents were azithromycin, cefaclor, cefixime, cefpodoxime, cefuroxime, ciprofloxacin, clarithromycin and loracarbef (100% each); the least active was ampicillin. Against penicillin-sensitive and -resistant pneumococci, the activity was not significantly different for azithromycin and clarithromycin (93.4-100%) and ciprofloxacin (MIC90 2.0 and 1.5 mg/L, respectively) but was different for cefuroxime (99.3% and 31.6%, respectively), cefaclor (MIC90 0.75 and > or = 256 mg/L, respectively), cefpodoxime (MIC90 0.047 and 1.5 mg/L, respectively) and loracarbef (MIC90 0.75 and > or = 256 mg/L, respectively). This study indicates the increasing incidence, in Canada, of beta-lactamase resistance in H. influenzae and M. catarrhalis and penicillin resistance in S. pneumoniae.

Canadian Pseudomonas aeruginosa susceptibility study from 48 medical centers: focus on ciprofloxacin.

We tested 1503 clinical isolates of Pseudomonas aeruginosa, from 48 Canadian medical centers, against ciprofloxacin and 11 other antimicrobial agents to determine in vitro activity. The frequency of susceptibility was highest for carbenicillin and ticarcillin (91% each) followed by imipenem and ceftazadime (90% each). Overall susceptibility (< or = 1.0 mg/l) to ciprofloxacin was 84% while resistance (> or = 4.0 mg/l) was 12%. Ciprofloxacin resistant isolates were more common from urinary tract specimens than from specimens collected from the respiratory and/or skin and soft tissue. Isolates from cystic fibrosis patients were more resistant to all agents tested than isolates from non-cystic fibrosis patients.