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.

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 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.

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.

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.

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.

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 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.

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.

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.

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.

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.

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.

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.

Sex differences favoring women in verbal but not in visuospatial episodic memory.

Sex differences favoring women have been found in a number of studies of episodic memory. This study examined sex differences in verbal, nonverbal, and visuospatial episodic memory tasks. Results showed that although women performed at a higher level on a composite verbal and nonverbal episodic memory score, men performed at a higher level on a composite score of episodic memory tasks requiring visuospatial processing. Thus, men can use their superior visuospatial abilities to excel in highly visuospatial episodic memory tasks, whereas women seem to excel in episodic memory tasks in which a verbalization of the material is possible.

Feasibility of SPECT for studies of brain perfusion during cardiopulmonary bypass.

Single-photon emission computed tomography (SPECT) and transcranial Doppler (TCD) ultrasonography were used to assess brain perfusion during cardiopulmonary bypass. Intravenous injections of technetium 99 m-hexamethylpropyleneamineoxime (99mTc-HMPAO) were administered before surgery and intraoperatively after the first 2 minutes in the first patient and at the end (42 minutes) of cardiopulmonary bypass in the second patient. The total middle cerebral artery territory counts were calculated using the region-of-interest method and compared to cerebellar regional counts. 99mTc-HMPAO uptake on SPECT scans was increased at the beginning and at the end of cardiopulmonary bypass, compared to baseline preoperative values (11-17%) in the presence of multiple microembolic signals on TCD (n1 = 35 and n2 = 42 for unilateral middle cerebral artery monitoring). These results indicate the feasibility of using HMPAO-SPECT to study brain perfusion changes during cardiac surgery. A combination of SPECT and TCD ultrasonography may be used to study the impact of microembolism during cardiac surgery with cardiopulmonary bypass.

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 role of germs and viruses in children's theories of AIDS (or, AIDS are not Band-Aids).

The development of knowledge of germs and viruses in relation to AIDS and flu was examined in a predominantly Mexican American sample of children aged 8-9, 10-11, and 12-13. Children progressed with age from identifying the disease agent for these diseases as a nondescript germ or something other than a germ to implicating a disease-specific germ or virus. Parallel age trends in mastery of the two diseases were observed; gender and ethnic differences were minimal. Solid command of germ and virus concepts in relation to AIDS was associated with more casually sophisticated understanding of the disease but not with more accurate knowledge of modes of HIV transmission. Grasp of flu germ/virus concepts did not contribute to greater understanding or knowledge of AIDS. Overall, children seem predisposed to construct a germ theory of an unfamiliar disease such as AIDS but need help in differentiating between one germ and another.

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 micrograms 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 microM)-induced protection of C8166 cells from 2.04 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 micrograms mL-1 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.