Na+/H+ antiport activity conferred by Bacillus subtilis tetA(L), a 5' truncation product of tetA(L), and related plasmid genes upon Escherichia coli

Site-directed mutagenesis studies of selected motif and charged residues and of cysteines of the multifunctional tetracycline efflux protein Tet(L)

All of the transmembrane glutamates of Tet(L) are essential for tetracycline (TET) resistance, and E397 has been shown to be essential for all catalytic modes, i.e., TET-Me(2+) and Na(+) efflux and K(+) uptake. Loop residues D74 and G70 are essential for TET flux but not for Na(+) or K(+) flux. A cysteineless Tet(L) protein exhibits all activities.

Twelve-transmembrane-segment (TMS) version (DeltaTMS VII-VIII) of the 14-TMS Tet(L) antibiotic resistance protein retains monovalent cation transport modes but lacks tetracycline efflux capacity

A "Tet(L)-12" version of Tet(L), a tetracycline efflux protein with 14 transmembrane segments (TMS), was constructed by deletion of two central TMS. Tet(L)-12 catalyzed Na+/H+ antiport and antiport with K+ as a coupling ion as well as or better than wild-type Tet(L) but exhibited no tetracycline-Me2+/H+ antiport in Escherichia coli vesicles.

Bacterial resistance and topical antimicrobial wash products

Current scientific evidence has not shown that a link exists between the use of topical antimicrobial formulations and antiseptic or antibiotic resistance. As a result of the extensive history and varied use of antiseptic products and ingredients, any selective pressure for antibiotic resistance that may be occurring or may be uncovered in the future because of antiseptic use would be expected to be insignificant compared with the selective pressure because of antibiotic use. This review illustrates the effectiveness of topical antimicrobial wash products against antibiotic-resistant and antiseptic-resistant bacteria in use settings as well as the studies performed (antiseptic, deodorant, and oral care) demonstrating the lack of development of resistance in long-term clinical studies. Although these studies illustrate that the use of topical antimicrobial products have not been shown to play a role in the fluctuations of the specific composition or resistance of the skin flora, changes in skin flora have been shown to occur. Based on current knowledge, the benefit from use of topical antimicrobial wash products in combination with standard infection control and personal hygiene practices far outweighs the risk of increased antibiotic resistance.

pH tolerance in Bacillus: alkaliphiles versus non-alkaliphiles

Monovalent cation/proton antiporters that catalyse electrogenic uptake of H+ in exchange for cytoplasmic K+ and/or Na+ are centrally involved in bacterial pH homeostasis under alkaline challenge. Systematic attempts have identified some, but not yet all, of the genes encoding such antiporters that participate in pH homeostasis in the neutrophilic Bacillus subtilis and the extremely alkaliphilic Bacillus firmus OF4. In each organism there are at least three distinct antiporters involved in pH homeostasis. They differ in cation requirement, with pH homeostasis specifically utilizing Na+/H+ antiport in the alkaliphile and using either Na+ or K+/H+ antiport in B. subtilis. Some of the antiporters involved in pH homeostasis are constitutive and are in place to respond to sudden pH shifts, but there is also an inducible component. At least two sets of homologous antiporters (NhaC and Mrp/Pha) function in both alkaliphiles and neutrophiles. An additional antiporter of a different transport protein family, the Gram-positive tetracycline-metal/H+ antiporter, is important in pH homeostasis in B. subtilis but has not yet been shown to be present in any alkaliphile. There are also differences outside of the antiporters themselves that contribute to the greater capacity of the alkaliphiles for pH homeostasis, including cation re-entry capacity and possible surface properties.

A putative multisubunit Na+/H+ antiporter from Staphylococcus aureus

We cloned several genes encoding an Na+/H+ antiporter of Staphylococcus aureus from chromosomal DNA by using an Escherichia coli mutant, lacking all of the major Na+/H+ antiporters, as the host. E. coli cells harboring plasmids for the cloned genes were able to grow in medium containing 0.2 M NaCl (or 10 mM LiCl). Host cells without the plasmids were unable to grow under the same conditions. Na+/H+ antiport activity was detected in membrane vesicles prepared from transformants. We determined the nucleotide sequence of the cloned 7-kbp region. We found that seven open reading frames (ORFs) were necessary for antiporter function. A promoter-like sequence was found in the upstream region from the first ORF. One inverted repeat followed by a T-cluster, which may function as a terminator, was found in the downstream region from the seventh ORF. Neither terminator-like nor promoter-like sequences were found between the ORFs. Thus, it seems that the seven ORFs comprise an operon and that the Na+/H+ antiporter consists of seven kinds of subunits, suggesting that this is a novel type of multisubunit Na+/H+ antiporter. Hydropathy analysis of the deduced amino acid sequences of the seven ORFs suggested that all of the proteins are hydrophobic. As a result of a homology search, we found that components of the respiratory chain showed sequence similarity with putative subunits of the Na+/H+ antiporter. We observed a large Na+ extrusion activity, driven by respiration in E. coli cells harboring the plasmid carrying the genes. The Na+ extrusion was sensitive to an H+ conductor, supporting the idea that the system is not a respiratory Na+ pump but an Na+/H+ antiporter. Introduction of the plasmid into E. coli mutant cells, which were unable to grow under alkaline conditions, enabled the cells to grow under such conditions.

Electrogenic antiport activities of the Gram-positive Tet proteins include a Na+(K+)/K+ mode that mediates net K+ uptake

Two Gram-positive Tet proteins, TetA(L) from Bacillus subtilis and TetK from a Staphylococcus aureus plasmid, have previously been suggested to have multiple catalytic modes and roles. These include: tetracycline (Tc)-metal/H+ antiport for both proteins (Yamaguchi, A., Shiina, Y., Fujihira, E., Sawai, T., Noguchi, N., and Sasatsu, M. (1995) FEBS Lett. 365, 193-197; Cheng, J. Guffanti, A. A., Wang, W., Krulwich, T. A., and Bechhofer, D. H. (1996) J. Bacteriol. 178, 2853-2860); Na+(K+)/H+ antiport for both proteins (Cheng et al. (1996)); and an electrical potential-dependent K+ leak mode for TetK and highly truncated segments thereof that can facilitate net K+ uptake (Guay, G. G., Tuckman, M., McNicholas, P., and Rothstein, D. M. (1993) J. Bacteriol. 175, 4927-4929). Studies of membrane vesicles from Escherichia coli expressing low levels of complete and 3'-truncated versions of tetA(L) or tetK, now show that the full-length versions of both transporters catalyze electrogenic antiport and that demonstration of electrogenicity depends upon use of a low chloride buffer for the assay. The K+ uptake mode, assayed via 86Rb+ uptake, was also catalyzed by both full-length TetA(L) and TetK. This mode does not represent a potential-dependent leak. Such a leak was not demonstrable in energized membrane vesicles. Rather, Rb+ uptake occurred in right-side-out vesicles when the intravesicular space contained either Na+ or K+ but not choline. If an outwardly directed gradient of Na+ or K+ was present, Rb+ uptake occurred without energization in vesicles from cells transformed with a plasmid containing tetA(L) or tetK but not a control plasmid. Experiments in which a comparable exchange was carried out in low chloride buffers to which oxonol was added confirmed that the exchange was electrogenic. Thus, the K+ uptake mode is proposed to be a mode of the electrogenic monovalent cation/H+ antiport activity of TetA(L) and TetK in which K+ takes the place of the external protons. Truncated TetK and TetA(L) failed to catalyze either Tc-metal/H+ or Na+/H+ antiport in energized everted vesicles. Truncated TetK, but not TetA(L), did, however, exhibit modest, electrogenic Na+(K+)/Rb+ exchange as well as a small, potential-dependent leak of Rb+. The C-terminal halves of the TetA(L) and TetK proteins are thus required both for proton-coupled active transport activities of the multifunctional transporter and, perhaps, for minimizing cation leakiness.

A Bacillus subtilis locus encoding several gene products affecting transport of cations

A 3.6-kb DNA fragment from Bacillus subtilis was found to complement the K+ uptake-deficient Escherichia coli strain TK2420. Transformation with a pKLO61 plasmid harboring this fragment conferred the capacity to grow on a minimal medium containing only 10 mM K+. Insertional mutagenesis and subcloning identified a single gene responsible for the complementation. This gene coded for an apparent homolog of E. coli TrkA. Sequence analysis of the cloned region also revealed three additional open reading frames. These included: a gene encoding a homolog to the czcD gene product of Alcaligenes eutrophus, a lysR-type regulatory gene which was found to enhance Na+ resistance in E. coli NM81 (delta nhaA) in a separate complementation test, and an orfD with no significant similarity to sequences deposited in Genbank.

The cost of antibiotic resistance--from the perspective of a bacterium

The possession of an antibiotic resistance gene clearly benefits a bacterium when the corresponding antibiotic is present. But does the resistant bacterium suffer a cost of resistance (i.e. a reduction in fitness) when the antibiotic is absent? If so, then one strategy to control the spread of resistance would be to suspend the use of a particular antibiotic until resistant genotypes declined to low frequency. Numerous studies have indeed shown that resistant genotypes are less fit than their sensitive counterparts in the absence of antibiotic, indicating a cost of resistance. But there is an important caveat: these studies have put antibiotic resistance genes into naïve bacteria, which have no evolutionary history of association with the resistance genes. An important question, therefore, is whether bacteria can overcome the cost of resistance by evolving adaptations that counteract the harmful side-effects of resistance genes. In fact, several experiments have shown that the cost of antibiotic resistance may be substantially diminished, even eliminated, by evolutionary changes in bacteria over rather short periods of time. As a consequence of this adaptation of bacteria to their resistance genes, it becomes increasingly difficult to eliminate resistant genotypes simply by suspending the use of antibiotics.

The purified Bacillus subtilis tetracycline efflux protein TetA(L) reconstitutes both tetracycline-cobalt/H+ and Na+(K+)/H+ exchange

Recent work has suggested that the chromosomally encoded TetA(L) transporter of Bacillus subtilis, for which no physiological function had been shown earlier, not only confers resistance to low concentrations of tetracycline but is also a multifunctional antiporter protein that has dominant roles in both Na(+)- and K(+)-dependent pH homeostasis and in Na+ resistance during growth at alkaline pH. To rigorously test this hypothesis, TetA(L) has been purified with a hexahistidine tag at its C terminus and reconstituted into proteoliposomes. The TetA(L)-hexahistidine proteoliposomes exhibit high activities of tetracycline-cobalt/H+, Na+/ H+, and K+/H+ antiport in an assay in which an outwardly directed proton gradient is artificially imposed and solute uptake is monitored. Tetracycline uptake depends on the presence of cobalt and vice versa, with the cosubstrates being transported in a 1:1 ratio. Evidence for the electrogenicity of both tetracycline-cobalt/H+ and Na+/H+ antiports is presented. K+ and Li+ inhibit Na+ uptake, but there is little cross-inhibition between Na+ and tetracycline-cobalt uptake activities. The results strongly support the conclusion that TetA(L) is a multifunctional antiporter. They expand the roster of such porters to encompass one with a complex organic substrate and monovalent cation substrates that may have distinct binding domains, and provide the first functional reconstitution of a member of the 14-transmembrane segment transporter family.

Proton-dependent multidrug efflux systems

Multidrug efflux systems display the ability to transport a variety of structurally unrelated drugs from a cell and consequently are capable of conferring resistance to a diverse range of chemotherapeutic agents. This review examines multidrug efflux systems which use the proton motive force to drive drug transport. These proteins are likely to operate as multidrug/proton antiporters and have been identified in both prokaryotes and eukaryotes. Such proton-dependent multidrug efflux proteins belong to three distinct families or superfamilies of transport proteins: the major facilitator superfamily (MFS), the small multidrug resistance (SMR) family, and the resistance/ nodulation/cell division (RND) family. The MFS consists of symporters, antiporters, and uniporters with either 12 or 14 transmembrane-spanning segments (TMS), and we show that within the MFS, three separate families include various multidrug/proton antiport proteins. The SMR family consists of proteins with four TMS, and the multidrug efflux proteins within this family are the smallest known secondary transporters. The RND family consists of 12-TMS transport proteins and includes a number of multidrug efflux proteins with particularly broad substrate specificity. In gram-negative bacteria, some multidrug efflux systems require two auxiliary constituents, which might enable drug transport to occur across both membranes of the cell envelope. These auxiliary constituents belong to the membrane fusion protein and the outer membrane factor families, respectively. This review examines in detail each of the characterized proton-linked multidrug efflux systems. The molecular basis of the broad substrate specificity of these transporters is discussed. The surprisingly wide distribution of multidrug efflux systems and their multiplicity in single organisms, with Escherichia coli, for instance, possessing at least nine proton-dependent multidrug efflux systems with overlapping specificities, is examined. We also discuss whether the normal physiological role of the multidrug efflux systems is to protect the cell from toxic compounds or whether they fulfil primary functions unrelated to drug resistance and only efflux multiple drugs fortuitously or opportunistically.