New quinolones and gram-positive bacteria

Susceptibility of ciprofloxacin-resistant staphylococci and enterococci to trovafloxacin

The susceptibilities of clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA) and enterococci obtained over a 13-year period were tested for trovafloxacin, a new fluoroquinolone, and ciprofloxacin. For MRSA, MIC50 values for trovafloxacin increased from 0.03 microgram/ml to 1 microgram/ml from 1984-1985 to 1995-1996, but were lower than those noted for ciprofloxacin, which increased from 0.25 microgram/ml to > 8 micrograms/ml during the same period. Trovafloxacin also showed lower MIC50 values (0.12-0.25 microgram/ml) than ciprofloxacin (MIC50 of 0.5-1 microgram/ml) for E. faecalis isolates from 1985 through 1991, but against E. faecium and E. faecalis isolated in 1995-1996, the MIC50 values for both trovafloxacin and ciprofloxacin were 2 micrograms/ml and the MIC90 values were > or 8 micrograms/ml. Thus, cross-resistance between fluoroquinolones was shown for both enterococci and MRSA. Whether the greater intrinsic activity of trovafloxacin might allow its use in treating MRSA and enterococcal infections remains to be seen.

Cloning and characterization of the parC and parE genes of Streptococcus pneumoniae encoding DNA topoisomerase IV: role in fluoroquinolone resistance

DNA topoisomerase IV mediates chromosome segregation and is a potential target for antibacterial agents including new antipneumococcal fluoroquinolones. We have used hybridization to a Staphylococcus aureus gyrB probe in concert with chromosome walking to isolate the Streptococcus pneumoniae parE-parC locus, lying downstream of a putative new insertion sequence and encoding 647-residue ParE and 823-residue ParC subunits of DNA topoisomerase IV. These proteins exhibited greatest homology respectively to the GrlB (ParE) and GrlA (ParC) subunits of S. aureus DNA topoisomerase IV. When combined, whole-cell extracts of Escherichia coli strains expressing S. pneumoniae ParC or ParE proteins reconstituted a salt-insensitive ATP-dependent decatenase activity characteristic of DNA topoisomerase IV. A second gyrB homolog isolated from S. pneumoniae encoded a 648-residue protein which we identified as GyrB through its close homology both to counterparts in S. aureus and Bacillus subtilis and to the product of the S. pneumoniae nov-1 gene that confers novobiocin resistance. gyrB was not closely linked to gyrA. To examine the role of DNA topoisomerase IV in fluoroquinolone action and resistance in S. pneumoniae, we isolated mutant strains stepwise selected for resistance to increasing concentrations of ciprofloxacin. We analysed four low-level resistant mutants and showed that Ser-79 of ParC, equivalent to resistance hotspots Ser-80 of GrlA and Ser-84 of GyrA in S. aureus, was in each case substituted with Tyr. These results suggest that DNA topoisomerase IV is an important target for fluoroquinolones in S. pneumoniae and establish this organism as a useful gram-positive system for resistance studies.

NorA plasmid resistance to fluoroquinolones: role of copy number and norA frameshift mutations

Staphylococcus aureus NorA protein is a transmembrane multidrug efflux pump that confers low-level resistance to hydrophilic fluoroquinolones. The norA gene promoter is active in Escherichia coli HB101. We have examined the genetic basis of norA-mediated resistance in E. coli by introducing a wild-type norA gene into HB101 in plasmid pCL1921, pBR322, or pUC18 exhibiting copy numbers that spanned a 22-fold range. Increased ciprofloxacin resistance correlated with norA transcript levels seen by Northern (RNA) analysis. Thus, contrary to some reports, a wild-type norA gene confers fluoroquinolone resistance in E. coli in a copy-number-dependent fashion and does not require mutational activation. Interestingly, a multicopy pUC19norA derivative gave transformants exhibiting a range of resistance phenotypes. The norA gene of one transformant carried a single base deletion (ATACAAT to AACAAT; the deleted base is underlined) in the putative--10 Pribnow box resulting in a promoter down-regulatory mutation; a second plasmid had acquired a frameshift producing a null mutation at codon 112. These mutations override the dual resistance-growth-inhibitory phenotype of high-copy-number norA plasmids. The results have implications for using the standard E. coli HB101 system to assess NorA function and potentially for plasmid-borne transmission of norA-mediated drug resistance.

In vitro and in vivo evaluations of LB20304, a new fluoronaphthyridone

In vitro activity of LB20304 against 1,231 clinical isolates was evaluated and compared with those of ciprofloxacin, sparfloxacin, lomefloxacin, and ofloxacin. LB20304 demonstrated the most potent activity against gram-positive bacteria. It was 32- to 64-fold more active than ciprofloxacin against methicillin-susceptible Staphylococcus aureus, methicillin-resistant S. aureus, methicillin-resistant Staphylococcus epidermidis, and Streptococcus pneumoniae (penicillin G resistant). LB20304 was also highly active against most members of the family Enterobacteriaceae. Its activity was more potent than those of sparfloxacin, ofloxacin, and lomefloxacin and comparable to that of ciprofloxacin. The protective activities of LB20304 against systemic infections caused by gram-positive bacteria in mice were superior to those of ciprofloxacin and sparfloxacin. Against infections by gram-negative bacteria, LB20304 was slightly less active than ciprofloxacin.

Fluoroquinolones in animal health

The fluoroquinolones are a series of synthetic antibacterial agents that are undergoing extensive investigation for both human and veterinary use in the treatment of a variety of bacterial infections. These agents work through the inhibition of DNA gyrase, interfering with the supercoiling of bacterial chromosomal material. As a result, these agents are rapidly bactericidal primarily against gram-negative bacteria, mycoplasma, and some gram-positive bacteria, with most having little to no activity against group D streptococci and obligate anaerobic bacteria. Resistance develops slowly and is almost always chromosomal and not plasmid-mediated. However, development of resistance to the fluoroquinolones and transfer of that resistance among animal and human pathogens have become a hotly debated issue among microbiologists. The fluoroquinolones are a current antimicrobial class whose use in veterinary medicine is being scrutinized. From a pharmacokinetic perspective, these agents are variably but well absorbed from the gastrointestinal tract and almost completely absorbed from parenteral injection sites, and they are well distributed to various tissues in the body. The fluoroquinolones are metabolized and renally excreted, with many of them having approximately equal excretion by the hepatic and the renal excretory systems. The primary toxicity observed at therapeutic doses involves the gastrointestinal system and phototoxicity, although at higher doses central nervous system toxicity and ocular cataracts are observed. Administration to immature animals may result in erosive arthropathies at weight-bearing joints, and administration of high doses to pregnant animals results in maternotoxicity and occasionally embryonic death. The fluoroquinolones are approved for indications such as urinary tract infections and soft tissue infections in dogs and cats and colibacillosis in poultry. Approval for bovine respiratory disease in the United States is being sought. Other indications for which the fluoroquinolones have been used in animal health include deep-seated infections, prostatitis, and other bacterial infections resistant to standard antimicrobial therapy.

Stomatococcus mucilaginosus septicemia in leukemic patients

OBJECTIVE: To report an unexpectedly high number of cases of septicemia with Stomatococcus mucilaginosus, and try to identify predisposing factors. METHODS: All blood cultures obtained during 1991--93 from patients treated at the hematologic ward were bacteriologically identified. The medical records of patients with S. mucilaginosus-positive blood cultures were retrospectively reviewed and evaluated. The antibiotic susceptibility pattern and restriction fragment length polymorphism (RFLP) of S. mucilaginosus were tested. RESULTS: S. mucilaginosus blood isolates from patients with hematologic malignancies were found to be as common as isolates of Staphylococcus aureus. Eleven patients with myelogenous leukemia and isolation of S. mucilaginosus from the blood are reported on. One patient had concomitant meningitis. All patients were neutropenic and most had oral mucositis and had been given ciprofloxacin prophylaxis. S. mucilaginosus isolates from these patients were resistant to ciprofloxacin in contrast to isolates from patients who had received other prophylactic regimens and seven isolates found in healthy individuals not recently treated with antibiotics. The resistant S. mucilaginosus were found to be of diverse genetic origin as determined by RFLP. CONCLUSIONS: The appearance of resistant strains during ciprofloxacin prophylaxis may be a predisposing factor for S. mucilaginosus septicemia. There was no evidence of a nosocomial spread of S. mucilaginosus strains.

Bactericidal effect of sparfloxacin alone and in combination with amoxicillin against Streptococcus pneumoniae as determined by kill-kinetic studies

Ten clinical isolates of Streptococcus pneumoniae (six intermediately penicillin-sensitive, one penicillin-resistant and three penicillin-sensitive strains) were studied by kill-kinetic experiments using sparfloxacin alone and in combination with amoxicillin. Equipotent doses of the antibiotics (1x, 2x and 4x the MIC) were used in the kill-kinetic studies. Sparfloxacin had a moderate bactericidal potential at 4x MIC after 5h and the combination with amoxicillin did not significantly increase its bactericidal activity. The clinical significance of these findings warrants further studies in vivo.

Current perspectives on glycopeptide resistance

In the last 5 years, clinical isolates of gram-positive bacteria with intrinsic or acquired resistance to glycopeptide antibiotics have been encountered increasingly. In many of these isolates, resistance arises from an alteration of the antibiotic target site, with the terminal D-alanyl-D-alanine moiety of peptidoglycan precursors being replaced by groups that do not bind glycopeptides. Although the criteria for defining resistance have been revised frequently, the reliable detection of low-level glycopeptide resistance remains problematic and is influenced by the method chosen. Glycopeptide-resistant enterococci have emerged as a particular problem in hospitals, where in addition to sporadic cases, clusters of infections with evidence of interpatient spread have occurred. Studies using molecular typing methods have implicated colonization of patients, staff carriage, and environmental contamination in the dissemination of these bacteria. Choice of antimicrobial therapy for infections caused by glycopeptide-resistant bacteria may be complicated by resistance to other antibiotics. Severe therapeutic difficulties are being encountered among patients infected with enterococci, with some infections being untreatable with currently available antibiotics.

Epidemiology of quinolone resistance: Europe and North and South America

After nearly 10 years of fluoroquinolone usage for a wide range of bacterial infections, a striking difference has been observed in the incidence of bacterial resistance to fluoroquinolones between bacteria responsible for community- and hospital-acquired infections, respectively. Resistance is only rarely encountered among common pathogens. In most studies, 97 to 100% of all pathogens are fully susceptible to fluoroquinolones. In contrast, resistance to fluoroquinolones has emerged and increased among bacteria responsible for nosocomial infections. The incidence of resistance to fluoroquinolones varies between bacterial species, clinical settings and countries, and is related to local epidemic spread of a few clones. The highest incidence of resistance is observed in Pseudomonas aeruginosa, Acinetobacter spp., Serratia marcescens and, particularly, methicillin-resistant Staphylococcus aureus (MRSA): some investigators have reported 95 to 100% fluoroquinolone resistance among MRSA. Follow-up of trends in the resistance to fluoroquinolones based upon surveillance programmes are needed.

Activity of quinolones against gram-positive cocci: clinical features

The potential role of the commercially available fluoroquinolones in the treatment of Gram-positive infections is discussed on the basis of data obtained from animal experiments and clinical trials. In respiratory tract infections, and particularly in community-acquired pneumonia, it is evident that the presently available quinolones cannot be prescribed empirically as first-line therapy because of their borderline activity against Streptococcus pneumoniae and anaerobes. Reports of pneumococcal seeding in other tissues during quinolone therapy render their administration a debatable issue. Experience in endocarditis is limited to the use of ciprofloxacin plus rifampicin in intravenous drug users with right-sided Staphylococcus aureus endocarditis. Patients with staphylococcal osteomyelitis are included among cases of other bone infections. In noncontrolled studies ciprofloxacin, ofloxacin and pefloxacin attained a staphylococcal eradication rate ranging from 70 to 100%, while the addition of rifampicin has been proven to reduce the emergence of resistant mutants during therapy. In soft tissue and skin structure infections that also involve Gram-negative bacteria, ciprofloxacin and ofloxacin eradicated 72.6 and 89% of staphylococci, respectively; however, the presence of diabetes or vascular disease compromised the success of treatment. In staphylococcal peritonitis complicating continuous ambulatory peritoneal dialysis, results with ciprofloxacin given intravenously or intraperitoneally were promising. In infections in neutropenic hosts, success of prophylaxis or therapy is still not clear, since colonisation and breakthrough bacteraemias with viridans streptococci and staphylococci have been reported. Furthermore, therapeutic results are compromised by the low response rate in Gram-positive infections. Despite the reported clinical efficacy of the newer fluoroquinolones, physicians should be alerted to the emergence of staphylococci resistant to fluoroquinolones, mainly methicillin-resistant variants.

Susceptibility of ciprofloxacin-resistant staphylococci and enterococci to clinafloxacin

Clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA) and Enterococcus faecalis obtained from the Ann Arbor Veterans Affairs Medical Center within the last decade were tested for susceptibility to ciprofloxacin and clinafloxacin. For MRSA isolates, the minimum inhibitory concentrations (MICs) of ciprofloxacin were several fold higher than those noted with clinafloxacin. Prior to the introduction of the fluoroquinolones (1984-1985), all MRSA isolates were susceptible to ciprofloxacin and clinafloxacin. By 1993, virtually all MRSA isolates were resistant to ciprofloxacin and a 50-fold increase in the MIC50 and MIC90 for clinafloxacin was seen. In 1985-1986, most enterococcal isolates were susceptible to ciprofloxacin and clinafloxacin. By 1993, one-third of all enterococci were resistant to both ciprofloxacin and clinafloxacin. Fluoroquinolone resistance developed more quickly in enterococci that demonstrated high-level gentamicin resistance. Thus, cross-resistance between clinafloxacin and ciprofloxacin was seen; however, the lower MICs of clinafloxacin for MRSA may allow the use of this drug for some MRSA infections.

In vitro activities of enoxacin, enrofloxacin, sparfloxacin, and ciprofloxacin against Escherichia coli strains isolated from diarrheic lambs and kids

The in vitro activities of four fluoroquinolone compounds were tested against 92 Escherichia coli strains of ovine and caprine origin under aerobic and anaerobic incubation conditions. The four fluoroquinolones proved to be highly effective against the E. coli isolates tested. When bacteria were cultured anaerobically, at least fourfold increases in the MICs of enoxacin for the strains occurred and no detectable changes in enrofloxacin, sparfloxacin, and ciprofloxacin MICs occurred.

Inactivation of antibiotics and the dissemination of resistance genes

The emergence of multidrug-resistant bacteria is a phenomenon of concern to the clinician and the pharmaceutical industry, as it is the major cause of failure in the treatment of infectious diseases. The most common mechanism of resistance in pathogenic bacteria to antibiotics of the aminoglycoside, beta-lactam (penicillins and cephalosporins), and chloramphenicol types involves the enzymic inactivation of the antibiotic by hydrolysis or by formation of inactive derivatives. Such resistance determinants most probably were acquired by pathogenic bacteria from a pool of resistance genes in other microbial genera, including antibiotic-producing organisms. The resistance gene sequences were subsequently integrated by site-specific recombination into several classes of naturally occurring gene expression cassettes (typically "integrons") and disseminated within the microbial population by a variety of gene transfer mechanisms. Although bacterial conjugation once was believed to be restricted in host range, it now appears that this mechanism of transfer permits genetic exchange between many different bacterial genera in nature.

Activity of Bay y3118 against quinolone-susceptible and -resistant gram-negative and gram-positive bacteria

The activity of Bay y3118 against laboratory strains of bacteria, including those with mutations in gyrA, with decreased expression of outer membrane proteins, and/or that are multiply resistant, and 121 selected clinical isolates, including highly fluoroquinolone-resistant bacteria from Spain and Argentina, was determined. Bay y3118 was extremely active (MICs, < or = 1 microgram/ml) against all bacteria, including quinolone-resistant laboratory strains. However, Bay y3118 was less active against 46 of 121 quinolone-resistant clinical isolates, such that > or = 16 micrograms of Bay y3118 per ml was required to inhibit 3 isolates. The concentration of Bay y3118 required to inhibit DNA synthesis by 50% correlated well with the MIC. Bay y3118 had accumulation kinetics similar to those of previously studied fluoroquinolones, e.g., ciprofloxacin, and there was a 50% decrease in the steady-state concentration in those members of the family Enterobacteriaceae that lacked porin proteins. Magnesium chloride at 20 mM apparently abolished the accumulation of Bay y3118 into Escherichia coli and reduced the level of accumulation into other gram-negative bacteria and Staphylococcus aureus. Carbonyl cyanide m-chlorophenylhydrazone at 100 microM enhanced the accumulation of Bay y3118 into E. coli, but it had a minimal effect on accumulation into S. aureus.