Streptococcus faecalis proton gradients and tetracycline transport

The lactococcal secondary multidrug transporter LmrP confers resistance to lincosamides, macrolides, streptogramins and tetracyclines

The active efflux of toxic compounds by (multi)drug transporters is one of the mechanisms that bacteria have developed to resist cytotoxic drugs. The authors describe the role of the lactococcal secondary multidrug transporter LmrP in the resistance to a broad range of clinically important antibiotics. Cells expressing LmrP display an increased resistance to the lincosamide, streptogramin, tetracycline and 14- and 15-membered macrolide antibiotics. The streptogramin antibiotic quinupristin, present in the fourth-generation antibiotic RP 59500, can inhibit LmrP-mediated Hoechst 33342 transport, but is not transported by LmrP, indicating that quinupristin acts as a modulator of LmrP activity. LmrP-expressing Lactococcus lactis cells in which a proton-motive force is generated accumulate significantly less tetracycline than control cells without LmrP expression. In contrast, LmrP-expressing and control cells accumulate equal amounts of tetracycline in the absence of metabolic energy. These findings demonstrate that the increased antibiotic resistance in LmrP-expressing cells is a result of the active extrusion of antibiotics from the cell.

Delta pH-dependent accumulation of tetracycline in Escherichia coli

The effects of ionophores on tetracycline accumulation in Escherichia coli cells were investigated in the presence of polymyxin B nonapeptide. Accumulation was inhibited by nigericin but not by valinomycin. Tetracycline accumulation was stimulated by decreasing the pH of the medium and inhibited by the addition of magnesium ions. These results indicated that tetracycline enters cells through diffusion as a protonated form (TH2) and is accumulated as a membrane-impermeable magnesium-tetracycline chelate complex (THMg+). This noncarrier diffusion hypothesis was confirmed by the fact that tetracycline accumulated in protein-free liposomes through an artificially imposed pH difference.

New perspectives in tetracycline resistance

Until recently, tetracycline efflux was thought to be the only mechanism of tetracycline resistance. As studies of tetracycline resistance have shifted to bacteria outside the Enterobacteriaceae, two other mechanisms of resistance have been discovered. The first is ribosomal protection, a type of resistance which is found in mycoplasmas, Gram-positive and Gram-negative bacteria and may be the most common type of tetracycline resistance in nature. The second is tetracycline modification, which has been found only in two strains of an obligate anaerobe (Bacteroides). Recent studies have also turned up such anomalies as a tetracycline efflux pump which does not confer resistance to tetracycline and a gene near the replication origin of a tetracycline-sensitive Bacillus strain which confers resistance when it is amplified.

Chloramphenicol accumulation by Haemophilus influenzae

The mechanism of chloramphenicol transport into susceptible strains of Haemophilus influenzae cells has not been reported previously. We examined apparent uptake of chloramphenicol by bacterial cells by using high-pressure liquid chromatography to quantitate drug disappearance from liquid media. Cell-associated chloramphenicol concentration is 1,000-fold greater than the extracellular drug concentration. Under incubation conditions associated with chloramphenicol disappearance from media, cellular protein synthesis was inhibited; however, if accumulation was inhibited, protein synthesis occurred in the presence of the drug. Chloramphenicol uptake appeared saturable (Km = 0.96 mM, Vmax = 0.9 mumol/min per mg of protein) and energy dependent: disappearance from media was markedly decreased by 2,4-dinitrophenol and carbonyl cyanide m-chlorophenylhydrazone, compounds which disrupt the proton motive force. Uptake occurred only in median which can support growth and was dependent upon temperature and pH. Drug accumulation was minimally affected by inhibitors of electron transport or by gentamicin and puromycin, both inhibitors of protein synthesis. The rate of disappearance was inhibited by SCH24893, a fluorinated chloramphenicol analog which also inhibits protein synthesis. We conclude that chloramphenicol accumulation by H. influenzae occurs by energy-dependent transport.

Biochemical and morphological properties of membranes of unsaturated fatty acid auxotrophs of Salmonella typhimurium: effects of fluorinated myristic acids

In order to investigate the utility of the fluorine-19 nucleus as a spectroscopic probe, a fluorinated analog of myristic acid has been incorporated into the membrane lipids of an unsaturated fatty acid auxotroph of Salmonella typhimurium. It is capable of supporting limited growth at temperatures above 37 degrees C. Freeze-fracture electron microscopic examinations of the membrane ultrastructure show a temperature and fatty acid supplement-dependent segregation of intramembranous protein particles into distinct patches in the auxotrophic membrane leaving intramembranous protein-denuded areas. The occurrence of these patches seems to be related to the phase separation of membrane lipids. Corresponding changes in the transport and accumulation of methyl thio-beta-D-galactopyranoside and tetracycline are observed. However, transport of histidine does not appear to be dependent on the physical state of the membrane lipids. The auxotroph shows differences in growth and morphological characteristics from those of the wild type. Functions of both inner and outer membranes are shown to be affected as a response to the fatty acid chain composition of the lipids.

Streptococcal tetracycline resistance mediated at the level of protein synthesis

The mechanism of tetracycline resistance was examined in strains containing each of the three previously identified resistance determinants in Streptococcus spp. Uptake of tetracycline was measured in tetracycline-sensitive cells as well as in cells containing each of the three resistance determinants. In cells containing tetL, uptake was not observed. However, in sensitive cells and cells containing either tetM or tetN, tetracycline was accumulated approximately 25-fold against a concentration gradient. Furthermore, there was no evidence for modification of intracellular tetracycline recovered from sensitive, tetM, or tetN cells. Protein synthesis in extracts derived from organisms containing tetM or tetN was resistant to tetracycline. In contrast, extracts of sensitive and tetL cells were sensitive to tetracycline.

Kinetic analysis of tetracycline accumulation by Streptococcus faecalis

Tetracycline transport by Streptococcus faecalis occurs by an energy-dependent, carrier-mediated process. The Michaelis constant for transport was unchanged, but the maximal velocity was increased when an energy source, glucose, was present. Accumulation levels, sometimes 25-fold greater than the external concentration, were controlled by the transport system.