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B M Howard

Publications and source records attributed to B M Howard.

13 recordsLinked to original sources
PubMed1994

Antagonism between bactericidal activities of 4-quinolones and coumarins gives insight into 4-quinolone killing mechanisms.

At concentrations exceeding their MICs, novobiocin and coumermycin antagonised the bactericidal activities of nalidixic acid, ciprofloxacin, ofloxacin and norfloxacin against Escherichia coli KL16. The sensitivities to killing by ciprofloxacin of four mutant derivatives of KL16 carrying gyrA, nalB, nal24 or nal31 alleles were also antagonised by novobiocin. The activities of drug combinations were tested in nutrient broth, which allowed expression of 4-quinolone killing mechanisms A, B and C. They were also tested in nutrient broth plus rifampicin to inhibit mechanisms A and C of the 4-quinolones, and in phosphate-buffered saline, which inhibited mechanism A. Results showed that novobiocin antagonised mechanism C, but not B, of both ciprofloxacin and ofloxacin, but did not antagonise mechanism C of norfloxacin. A review of these and other data indicates that mechanism B may result from the activities of SOS error-prone DNA repair on an irreversibly-bound drug-gyrase-DNA complex, and that mechanism C is mediated via drug interaction with the B subunit of DNA gyrase.

Aminocoumarins↗
PubMed1993

Function of the SOS process in repair of DNA damage induced by modern 4-quinolones.

The recA13 mutant of Escherichia coli strain K-12, which lacks recombination and SOS error-prone DNA repair is hypersensitive to nalidixic acid and to the newer 4-quinolones ciprofloxacin, norfloxacin and ofloxacin. However, whereas recombination-proficient but SOS repair-deficient strains, such as those carrying the lexA3 or recA430 alleles are no more sensitive to nalidixic than the lexA+ recA+ parent, they are more sensitive to the newer quinolones, although not as sensitive as the recA13 derivative. Nalidixic acid possesses only bactericidal mechanism A (which requires RNA and protein synthesis and is only effective on actively dividing cells), whereas the newer 4-quinolones exhibit additional mechanisms B (which does not require RNA and protein synthesis and is effective on bacteria unable to multiply) and C (which requires RNA and protein synthesis but does not depend on cell division). Results obtained with bacteria suspended in phosphate-buffered saline, which inhibits mechanism A, and with bacteria suspended in nutrient broth plus rifampicin, which inhibits mechanisms A and C, showed that the lexA3 mutant was still more sensitive than the lexA+ parent under these conditions. The results suggest that, unlike bactericidal mechanism A, DNA damage that results from bactericidal mechanisms B and C of the newer 4-quinolones is subject to SOS error-prone (mutagenic) repair.

Anti-Infective Agents↗
PubMed1993

Contributions of post-antibiotic lag and repair-recovery to the post-antibiotic effects of ciprofloxacin on Escherichia coli, Klebsiella pneumoniae, Staphylococcus aureus and Streptococcus pyogenes.

A viable counting technique was used to determine the post-antibiotic effect (PAE) of ciprofloxacin against four bacterial species, treated with either one or four times the minimum inhibitory concentration for 1 or 3 h. PAE were demonstrated with Escherichia coli, Staphylococcus aureus or Streptococcus pyogenes after exposure to either concentration for both times. No clear PAE was demonstrated for Klebsiella pneumoniae after any treatment. In some cases, PAE was due to an increased post-treatment lag phase, which was followed by nearly normal multiplication, whereas in other cases a long lag was followed by abnormally rapid cell division, with the generation times of treated cultures being much less than those of the corresponding drug-free controls. This is evidence of recovery of cells that have completed DNA repair. S. aureus, E. coli and K. pneumoniae all exhibited evidence of this type of repair even though K. pneumoniae gave no significant PAE. However, the post-treatment generation times of S. pyogenes, which produced the greatest PAE, gave no evidence of such repair. It is concluded that PAEs may result from a variety of factors.

Ciprofloxacin↗
PubMed1993

4-Quinolone bactericidal mechanisms.

The bactericidal activity of nalidixic acid against Escherichia coli strain KL16 in nutrient broth was abolished by the addition of rifampicin. Cells suspended in phosphate-buffered normal saline (PBS) were also not killed by nalidixic acid. Experiments with modern 4-quinolones showed their activities varied according to the conditions under which they were tested. Rifampicin did not affect the concentration at which ofloxacin became bactericidal in nutrient broth, but did limit the extent of ofloxacin-induced death. However, rifampicin produced a 10-fold increase in the concentration at which ciprofloxacin became bactericidal in nutrient broth, and completely abolished the bactericidal activity of norfloxacin. Unlike nalidixic acid all of the modern 4-quinolones killed cells suspended in PBS. Based on these results it was possible to differentiate 3 processes by which 4-quinolones induced death. Mechanism A was only active against dividing bacteria and required RNA and protein synthesis; it was therefore not active against bacteria suspended in PBS and was inhibited in nutrient broth by the addition of rifampicin. Mechanism B required neither RNA nor protein synthesis and was also active against non-dividing bacteria; it was therefore not inhibited by rifampicin nor by suspending bacteria in PBS. Mechanism C killed non-dividing bacteria, but required protein and RNA synthesis: it therefore functioned in PBS, but was inhibited by rifampicin.(ABSTRACT TRUNCATED AT 250 WORDS)

4-Quinolones↗
PubMed1993

Studies on mutational cross-resistance between ciprofloxacin, novobiocin and coumermycin in Escherichia coli and Staphylococcus warneri.

Nalidixic acid resistant mutants of Escherichia coli KL16 were tested against ciprofloxacin, coumermycin and novobiocin. The mutants gyrA, nalB and nal-24 were more resistant than KL16 to ciprofloxacin, whereas the nal-31 strain was hypersensitive. Only the nalB mutant was more resistant to novobiocin than KL16, but gyrA, nal-31 and nal-24 mutants were more sensitive to coumermycin than KL16. Newly-isolated novobiocin-resistant mutants of KL16 were not cross-resistant to coumermycin or ciprofloxacin. Some coumermycin-resistant mutants were cross-resistant to novobiocin but not ciprofloxacin, whereas mutants resistant to novobiocin and ciprofloxacin were isolated at higher coumermycin concentrations. Two types of Staphylococcus warneri mutant were isolated on media containing novobiocin or coumermycin. Each was resistant to both coumarins, but one was highly resistant to novobiocin and the other to coumermycin. High level resistance to both coumarins was unstable. E. coli mutants differed in susceptibility to bactericidal concentrations of ciprofloxacin, and S. warneri mutants behaved similarly. These results suggest the modes of action of the coumarins are not identical.

Aminocoumarins↗
PubMed1991

Protein- and RNA-synthesis independent bactericidal activity of ciprofloxacin that involves the A subunit of DNA gyrase.

Ciprofloxacin, unlike nalidixic acid, can kill Escherichia coli cells in the absence of synthesis of protein or RNA. Hence, chloramphenicol or rifampicin do not abolish the bactericidal activity of ciprofloxacin against wild-type E. coli. Protein and RNA synthesis were not required for the bactericidal activity of ciprofloxacin against nalB, nalC and nalD mutants of E. coli. However, the addition of chloramphenicol or rifampicin abolished the bactericidal activity of ciprofloxacin against a nalA mutant in nutrient broth. It is concluded that the ability of ciprofloxacin to kill E. coli in the absence of protein or RNA synthesis involves the A subunit of DNA gyrase.

Cell Membrane Permeability↗
PubMed1991

4-Quinolone interactions with gyrase subunit B inhibitors.

In studies which have involved determination of fractional inhibitory concentrations, synergy has been described between the 4-quinolones, which inhibit the A subunit of DNA gyrase, and either coumermycin or novobiocin, which inhibit the B subunit of the same enzyme. In this study, fixed concentrations of ciprofloxacin or ofloxacin were combined with varying concentrations of coumermycin or novobiocin and vice versa in nutrient broth. The bactericidal activities of the different mixtures against either Staphylococcus aureus E3T or S. warneri were determined and found to be less than those of equivalent concentrations of either 4-quinolone alone. The observation that gyrase B subunit inhibitors antagonised the bactericidal activity of 4-quinolones is in accordance with the report previously made by others that ciprofloxacin combined with coumermycin was less effective than ciprofloxacin alone in treating staphylococcal endocarditis in rats. Our results indicate that both inhibitory and bactericidal activity should be taken into account when assessing possible interactions in vivo between 4-quinolones and other antimicrobial agents.

Aminocoumarins↗
PubMed1989

4-quinolones and the SOS response.

The SOS DNA repair system is induced in bacteria treated with 4-quinolones. However, whether the response exacerbates or repairs the damage caused by these drugs is still unclear. The recA13 and the recB21 mutations impair recombination repair and render bacteria unable to induce the SOS response when treated with nalidixic acid or other agents that affect DNA synthesis. However, UV treatment induces the SOS response in recB21 mutants but not in recA13 mutants. Both these mutants are hypersensitive to nalidixic acid and, therefore, either recombination repair or SOS repair would appear to repair DNA damage caused by the drug. However, since the lexA3 mutation (which also renders bacteria incapable of inducing the SOS response without affecting recombination repair) had no effect on the susceptibility of bacteria to nalidixic acid, the SOS response neither contributes to nor repairs DNA damage caused by the drug. Consequently, it would seem that the hypersensitivity of the recA13 and recB21 mutants to nalidixic acid is due to their deficiency in recombination repair. This view was confirmed by testing a recA430 mutant that is recombination-repair proficient but SOS repair-deficient and finding it to be no more sensitive to nalidixic acid than its parent. Thus it would appear that, although induced by nalidixic acid treatment, the SOS DNA repair system does not play any role in bacterial responses to the damage caused by the drug. In contrast, the recombination repair system does repair damage caused by nalidixic acid.

Anti-Infective Agents↗
PubMedDate not supplied

Post-antibiotic effects of ofloxacin on Escherichia coli, Klebsiella pneumoniae, Staphylococcus aureus, and Streptococcus pyogenes.

A viable counting technique was used to determine the post-antibiotic effect (PAE) of ofloxacin against four bacterial species, treated with either once of four times the minimum inhibitory concentration for 1 or 3 h. Similar to the results obtained previously with ciprofloxacin, ofloxacin gave PAE values with Escherichia coli, Staphylococcus aureus, and Streptococcus pyogenes. Cell division of Klebsiella pneumoniae was inhibited on removal of ofloxacin, but no clear PAE was demonstrated with this species because once replication recommenced, the mean generation times of drug-treated cultures were much shorter than those of untreated controls. Therefore, although the results obtained with ciprofloxacin and ofloxacin imply a consistency of PAE for 4-quinolones within a species, the response to DNA damage induced by 4-quinolones is multifaceted and species dependent. 4-quinolones inhibit both DNA replication and cell division, whilst at the same time stimulating DNA repair pathways. Thus, in some cases PAEs result from an increased post-treatment lag phase which may be followed by nearly normal multiplication, whereas in other cases a long lag may be followed by abnormally rapid cell division, with the generation times of treated cultures being shorter than those of corresponding drug-free controls. The PAE of a drug-induced lag may thus be masked by rapid cell division once growth resumes.

Colony Count, Microbial↗
PubMedDate not supplied

Post-antibiotic effects of cefdinir on Escherichia coli, Klebsiella pneumoniae, Staphylococcus aureus and Streptococcus pyrogenes.

The post-antibiotic effects (PAEs) of a new cephalosporin, cefdinir, were determined against a range of organisms using a viable counting technique. Cefdinir exerted considerable PAEs against Staphylococcus aureus and Streptococcus pyogenes, but no overall post-antibiotic inhibition of growth was detected against Escherichia coli or Klebsiella pneumoniae. Exposure to cefdinir made the gram-negative organisms susceptible to the washing procedure used for drug removal, but this was followed by rapid recovery of viability in drug-free broth.

Cefdinir↗