The effect of thymidine on the antibacterial and antiviral activity of zidovudine

The effect of thymidine and deoxyadenosine on the antiviral and antibacterial effect of zidovudine was studied in human immunodeficiency virus type 1 (HIV-1) Escherichia coli and Salmonella typhimurium. In quantitative assays, 10 μg mL−1 thymidine was shown to increase the 50% inhibitory concentration (IC50) of zidovudine for HIV-1 by approximately 100-fold and to reduce zidovudine (1 μm)-induced protection of C8166 cells from 204 to 0·18 log syncytial-forming units. Thymidine also antagonized the antibacterial effect of zidovudine for two E. coli and three S. typhimurium species in a dose-dependent manner; 10 μg mL of thymidine increased the minimum inhibitory concentration of zidovudine for E. coli strains by 10–40-fold and for S. typhimurium strains by three-fold. Deoxyadenosine reduced the minimum inhibitory concentration of zidovudine against all five bacterial strains but had no effect on the IC50 of zidovudine for HIV-1, nor did it significantly reverse the antagonism of the antibacterial and antiviral activity of thymidine. The induction of the SOS response in E. coli was reversed in a dose-dependent manner by thymidine while the presence of deoxyadenosine increased induction of the SOS response by zidovudine at suboptimal concentrations.

Mechanisms of antimicrobial resistance and implications for epidemiology

The development of antibacterial agents has provided a means of treating bacterial diseases which were, previously, often fatal in both man and animal and thus represents one of the major advances of the 20th century. However, the efficacy of these agents is increasingly being compromised by the development of bacterial resistance to the drugs currently available for therapeutic use. Bacterial resistance can be combated in two ways. New drugs to which bacteria are susceptible can be developed and policies to contain the development and spread of resistance can be implemented. Both strategies require an understanding of the mechanisms of drug resistance, its epidemiology and the role of environmental factors in promoting resistance. Over the past thirty years our knowledge of bacterial resistance has increased dramatically mainly due to new technology that has become available. Bacteria are able to resist antibacterials by a variety of mechanisms: for example, altering the target to decrease susceptibility to the antibacterial, inactivating or destroying the drug, reducing drug transport into the cell or metabolic bypass. These drug resistance determinants are mediated via one of two distinct genetic mechanisms, a mutation in the bacterial chromosome or by a transmissible element; either a plasmid or a transposon. Significant differences exist between these two types of drug resistance as transmissible resistance, which is mainly plasmid-mediated, permits intraspecies and even interspecies transfer to occur. In contrast, chromosomal resistance can only be passed on to progeny. Transmissible antibacterial resistance is the major cause of concern as it can lead to the rapid spread of antibacterial resistance and has proven difficult, if not impossible, to eradicate. Furthermore, plasmids and transposons can code for multiple antibiotic resistance as well as virulence genes. Antibacterials for which transferable resistance has been identified include most commonly used antibacterials such as beta-lactams, aminoglycosides, macrolides, sulphonamides, tetracyclines, chloramphenicol and trimethoprim. One notable exception is the 4-quinolones for which plasmid-mediated resistance has yet to be identified.

The bactericidal activity of sparfloxacin

Sparfloxacin was found to display a biphasic response against Escherichia coli, Staphylococcus aureus and Staphylococcus epidermidis in nutrient broth. Its optimum bactericidal concentration was found to be identical for all three species which contrasts with other clinically available fluoroquinolones that are more active against E. coli than against staphylococci. Bacterial protein and RNA synthesis as well as cell division were not found to be essential for all the lethality of sparfloxacin, which hence displays bactericidal mechanism B. However, sparfloxacin was unable to kill bacteria in absence of oxygen.

Antibacterial activity of a 1,8-naphthyridine quinolone, PD 131628

A new 1,8-naphthyridine quinolone antimicrobial, PD131628, was found to be as, or more active than ciprofloxacin or ofloxacin against Escherichia coli and considerably more active than the two 4-quinolone agents against staphylococci in terms of both MIC and OBC (the optimum bactericidal concentration, at which the rate of kill is greatest). In common with ciprofloxacin and ofloxacin, the rate of kill of PD131628 against Enterococcus faecalis was considerably slower than against the other three species tested. Bacterial protein synthesis, RNA synthesis and cell division were not required for the bactericidal activity of PD131628 against E. coli or the staphylococci, although the lethality of PD131628 in the absence of these activities was reduced.

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.

In vitro susceptibility of the fish pathogen Aeromonas salmonicida to flumequine

The activity of the fluoroquinolone flumequine was investigated against the fish pathogen Aeromonas salmonicida and was compared with that of oxolinic acid. Flumequine was more active than oxolinic acid in terms of its MIC against oxolinic acid-resistant isolates of A. salmonicida and was as active as oxolinic acid against susceptible isolates. In contrast to oxolinic acid, flumequine was bactericidal, with only 1% of the bacteria surviving 6 h of exposure to the drug at concentrations slightly above the MIC. Mutation to resistance to flumequine was found to occur at a lower frequency than that to oxolinic acid. Hence, in vitro, flumequine appears to possess some advantages over oxolinic acid against this fish pathogen.

The fluoroquinolones exert a reduced rate of kill against Enterococcus faecalis

The bactericidal activities of ciprofloxacin, ofloxacin and DR-3355 have been investigated against Enterococcus faecalis, Staphylococcus aureus and Staphylococcus epidermidis over 24 h. The three fluoroquinolones were found to be rapidly bactericidal against the staphylococci, killing over 99% of the bacteria during the first 3 h of exposure with a further reduction in viability of approximately one logarithm occurring over the next 21 h. In contrast, the fluoroquinolones displayed a much slower rate of kill against E. faecalis, as little or no bactericidal activity was detected over the first 3 h for both E. faecalis ATCC19433 and a clinical isolate. At 6 h all three of the drugs were bactericidal against the enterococci although the amount of kill was not as great as against the staphylococci. However, at 24 h the amount of kill obtained with all three drugs was similar to that obtained for staphylococci exposed to these drugs. Ciprofloxacin, ofloxacin and DR-3355 were not active against E. faecalis ATCC19433 in phosphate buffered saline and therefore require cell division for their bactericidal activity against this species.

The role of the SOS response in bacteria exposed to zidovudine or trimethoprim

Trimethoprim was more potent than zidovudine as an inducer of the SOS response in Escherichia coli. The level of induction by each compound initially increased with rising drug concentration and then fell; this effect was less marked with zidovudine than with trimethoprim. The SOS response did not appear to be involved in the inhibition of bacterial multiplication as the MICs of trimethoprim or zidovudine for recA430 and lexA3 mutants, which are unable to induce the SOS response, were identical to the MICs for the parent strains. However, the bactericidal activity of each compound against strains deficient in the SOS response was reduced. This suggest that induction of the DNA repair system contributes to the bactericidal activity of the drugs.

The mode of action of quinolones: the paradox in activity of low and high concentrations and activity in the anaerobic environment

All 4-quinolones that have been examined display rapid bactericidal activity which is biphasic. At concentrations above the MIC, the lethality of the drugs increases until a concentration known as the optimum bactericidal concentration (OBC) beyond which the bactericidal activity then declines. The biphasic response appears to be due to the inhibition of RNA synthesis at concentrations above the OBC, as RNA synthesis is required for the full bactericidal activity of the 4-quinolones. However, differences in the biphasic response are observed as some fluoroquinolones are still able to kill bacteria in the absence of bacterial protein or RNA synthesis, thus reducing the inhibition of bactericidal activity at concentrations above the OBC. It has been proposed that this ability to kill bacteria in the absence of protein or RNA synthesis is due to the possession of an additional bactericidal mechanism by these fluoroquinolones. Oxygen also appears to be essential for the lethality of the clinically available 4-quinolones although it is not required for the drugs to inhibit bacterial multiplication. Therefore these drugs are not bactericidal under anaerobic conditions.

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.

Mechanisms of zidovudine resistance in bacteria

Unlike their parent strains, zidovudine-resistant derivatives of Escherichia coli KL16 and Salmonella typhimurium NCTC 5710 were found to be incapable of incorporating radiolabelled thymidine into their chromosomal DNA. Since incorporation was still prevented in the presence of EDTA, resistance to zidovudine was not associated with a permeability barrier, but appeared to result from the loss of thymidine kinase activity, required for the phosphorylation of zidovudine. Pseudomonas aeruginosa, which is intrinsically zidovudine-resistant, was also shown to be incapable of incorporating thymidine into its DNA, but Staphylococcus epidermidis SK360 and Staph. aureus E3T, which are also intrinsically zidovudine-resistant, possessed thymidine kinase activity. This suggests that two distinct mechanisms of resistance to zidovudine exist in bacteria. Zidovudine resistance did not appear to confer resistance to other antibacterial agents.

Susceptibility of bacterial isolates from AIDS patients to six fluoroquinolones

The susceptibilities to ciprofloxacin, DR-3355 (S-(-)-ofloxacin), enoxacin, lomefloxacin, ofloxacin and PD127,391 of 69 significant bacterial isolates from HIV-positive patients at the City Hospital, Edinburgh have been determined. With the exception of the enterococci, most of the strains tested (including staphylococci, Escherichia coli and Pseudomonas aeruginosa) were susceptible to the fluoroquinolones. Ciprofloxacin was the most active of the clinically available drugs followed by ofloxacin, lomefloxacin and enoxacin. PD127,391 and DR-3355, the new fluoroquinolones tested, were at least as active as ciprofloxacin. Hence bacterial infections in AIDS patients should respond to fluoroquinolone therapy.

Cross resistance between oxytetracycline and oxolinic acid in Aeromonas salmonicida associated with alterations in outer membrane proteins

Oxytetracycline resistant mutants of Aeromonas salmonicida isolated from mutation frequency experiments showed decreased susceptibility to oxolinic acid. Outer membrane preparations of these resistant mutant strains revealed a major protein, with a molecular mass of approximately 37 kDa, which was not present in significant quantities in the parent strain.

Antibacterial activity of fluoroquinolones in combination with zidovudine

Since patients with AIDS may receive fluoroquinolones concurrently with zidovudine, the antibacterial interaction of these drugs was investigated. No evidence was found of antagonism between zidovudine and ciprofloxacin, DR-3355, enoxacin, lomefloxacin or ofloxacin against enterobacteria, staphylococci or Pseudomonas aeruginosa. Furthermore, the bactericidal activity of the fluoroquinolones against selected enterobacteria in nutrient broth was not affected by a clinically achievable concentration of zidovudine. It seems unlikely that zidovudine will have an adverse effect on the antibacterial activity of the fluoroquinolones in patients with AIDS.

Assessment of the interaction between ciprofloxacin and teicoplanin in vitro and in neutropenic patients

The interactions of ciprofloxacin and teicoplanin were investigated against methicillin-sensitive and methicillin-resistant Staphylococcus aureus and Staph. epidermidis isolated during a clinical trial of the efficacy of this combination. In-vitro studies of the combination of ciprofloxacin and teicoplanin found no evidence of any antagonism between these two drugs in terms of inhibition of bacterial multiplication, as determined by Fractional Inhibitory Concentration Indices, and lethality. Clinical use of teicoplanin plus ciprofloxacin as empirical therapy in 29 febrile neutropenic patients revealed an overall response rate of 75%. Response rate for staphylococcal infections, which accounted for 53% of isolated pathogens, was 80%. No serious adverse drug reactions were seen. Our results show that both in vitro, and in the treatment of febrile neutropenic patients, teicoplanin plus ciprofloxacin is an effective anti-staphylococcal combination.

Bactericidal action of PD127,391, an enhanced spectrum quinolone

The 4-quinolone PD127,391 displays a biphasic effect on Escherichia coli, Staphylococcus aureus and S. epidermidis in nutrient broth. It is as active as ciprofloxacin in terms of its optimum bactericidal concentration against E. coli. However, against staphylococci it is six times as active as ciprofloxacin or any other 4-quinolone previously investigated. Although protein and RNA synthesis are not required for bactericidal activity, cell division is essential.

In vitro activities of 4-quinolones against the fish pathogen Aeromonas salmonicida

The activities of five fluorinated 4-quinolones, namely, sarafloxacin, enrofloxacin, PD127391, PD117596, and CI934, against the fish pathogen Aeromonas salmonicida were investigated and compared with that of oxolinic acid. The results indicated that with the exception of CI934, these drugs are more active than oxolinic acid in terms of MIC. No inoculum effect was observed, but the drugs were less active at 10 degrees C than at 22 degrees C. The presence of 3% of NaCl caused an increase in drug activity. Resistance to the drugs appeared to be fairly stable, with only a small decrease in activity after 10 successive passages of the test strains on drug-free tryptone soya agar.

Conditions required for the bactericidal activity of 4-quinolones against Serratia marcescens

The conditions required to kill Serratia marcescens with nalidixic acid, ciprofloxacin, norfloxacin or ofloxacin were determined in nutrient broth and in phosphate-buffered saline. They were found to be similar to the conditions required for these 4-quinolones to kill Escherichia coli. Bacterial RNA synthesis and bacterial cell division were essential for the bactericidal activity of nalidixic acid but all three fluoroquinolones were bactericidal against non-dividing S. marcescens. However, as with E. coli, bacterial RNA synthesis was essential for the bactericidal activity of norfloxacin though this was not required to kill S. marcescens with ciprofloxacin or ofloxacin.

Isolation of zidovudine resistant Escherichia coli from AIDS patients

Zidovudine-resistant Escherichia coli were isolated from faecal samples from 6 out of 11 AIDS patients receiving zidovudine. Resistance appeared to be due to the loss of thymidine kinase activity which is required for the phosphorylation of zidovudine to its active form. No zidovudine resistant enterobacteria were isolated from 30 control faecal samples. Hence, clinically, there appeared to be a high correlation between the development of zidovudine-resistance in E. coli and exposure to zidovudine (chi 2: 11.77, P less than 0.001). However the development of zidovudine resistance does not appear to be associated with cross-resistance to other antimicrobial agents as the zidovudine-resistant E. coli did not display a high degree of resistance to other antibacterials.

Conditions required for the bactericidal activity of fleroxacin and pefloxacin against Escherichia coli KL 16

Pefloxacin and fleroxacin showed biphasic bactericidal activity against Escherichia coli KL16 in nutrient broth. Bacteriostatic concentrations of chloramphenicol, an inhibitor of protein synthesis, and rifampicin, an inhibitor of RNA synthesis, could not completely abolish the bactericidal activity of either drug. Pefloxacin and fleroxacin were also active against non-dividing E. coli KL16. Therefore, pefloxacin and fleroxacin are able to kill bacteria which are not dividing nor actively synthesising protein or RNA.

DNA breakdown by the 4-quinolones and its significance

DNA breakdown occurred in Escherichia coli KL16 exposed to nalidixic acid, ciprofloxacin or norfloxacin. However DNA breakdown does not seem to be the cause of the lethality of the 4-quinolones because it still occurred under conditions which abolished the lethality of nalidixic acid. Furthermore, no correlation was found between the amount of DNA breakdown and the rate of death of bacteria caused by the three 4-quinolones. Similarly, DNA breakdown did not occur when recB or recC mutants were treated with nalidixic acid despite both mutants being killed by the drug, again suggesting that DNA breakdown is not the cause of bacterial death. Since recB and recC mutants lack exonuclease V, this enzyme may be responsible for the DNA breakdown observed in bacteria treated with 4-quinolones.

The bactericidal activity of DR-3355, an optically active isomer of ofloxacin

The bactericidal activity of compound DR-3355, an optically active isomer of ofloxacin, was measured against Escherichia coli, Staphylococcus aureus and S. epidermidis, in nutrient broth and in phosphate-buffered saline. DR-3355 was found to be approximately twice as active as ofloxacin in terms of the concentration at which maximum bacterial kill was achieved. Hence it appears that DR-3355 is twice as active as ofloxacin not only in terms of its ability to inhibit bacterial multiplication but also in its ability to kill bacteria. DR-3355 was found to be active against non-dividing bacteria and did not require either active RNA or protein biosynthesis in order to kill bacteria.

Conditions required for the antibacterial activity of zidovudine

In a study to determine the conditions required for its antibacterial activity, zidovudine was found to be bactericidal against Escherichia coli and Salmonella typhimurium in nutrient broth over 6 h. Zidovudine was also found to be active against ten clinical isolates of Salmonella and Escherichia coli at 1 mg/l, which is a clinically achievable concentration of the drug. Bacterial protein synthesis was required for the bactericidal activity of zidovudine against both Escherichia coli and Salmonella typhimurium. Zidovudine was found to be inactive against non-dividing bacteria, as it was unable to kill bacteria of either species when they were suspended in PBS.

Bactericidal activity of enoxacin and lomefloxacin against Escherichia coli KL16

Enoxacin and lomefloxacin were found to display a biphasic response when their bactericidal activities were investigated against Escherichia coli KL16 in nutrient broth. Although enoxacin required bacterial protein and RNA synthesis to exert bactericidal activity, it was able to kill non-dividing bacteria. On the other hand, the protein synthesis inhibitor chloramphenicol and the RNA synthesis inhibitor rifampicin did not abolish enoxacin's killing activity against Escherichia coli KL16 in nutrient broth. Lomefloxacin was also shown to be active against non-dividing Escherichia coli KL16.

Interactions of the 4-quinolones with other antibacterials

The effect of sub-inhibitory concentrations of 16 antibacterials on the bactericidal activity of the 4-quinolones nalidixic acid, ciprofloxacin and ofloxacin against Escherichia coli KL16 in nutrient broth was investigated. Sub-inhibitory concentrations of rifampicin, clindamycin, chloramphenicol, erythromycin or tetracycline antagonised the bactericidal activity of the 4-quinolones. Conversely, all seven aminoglycosides tested enhanced the bactericidal activity of the 4-quinolones whereas the cell wall antagonists, azlocillin, mezlocillin, ceftazidime and vancomycin had no effect on the bactericidal activity of the 4-quinolones.

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.

Bactericidal mechanisms of ofloxacin

Ciprofloxacin and ofloxacin are known to exert a second bactericidal mechanism (termed B) against Escherichia coli which functions even when protein synthesis is inhibited by chloramphenicol or when RNA synthesis is inhibited by rifampicin. However, the bactericidal activity of ciprofloxacin against a coagulase-negative staphylococcus (Staphylococcus warneri) was found to be abolished by chloramphenicol so the 4-quinolone does not exert mechanism B against this species. On the other hand, ofloxacin did exhibit mechanism B against S. warneri because the drug remained bactericidal in the presence of chloramphenicol. When S. aureus was investigated results similar to those observed in S. warneri were obtained throughout the range of clinically achievable concentrations of ofloxacin and ciprofloxacin. Ofloxacin seems to exhibit mechanism B against the staphylococci while ciprofloxacin does not. This may explain why ciprofloxacin is more potent than ofloxacin against Gram-negative bacteria but against staphylococci both drugs are equipotent.