Decreased membrane permeability in a polymyxin B-resistant Escherichia coli mutant exhibiting multiple resistance to beta-lactams as well as aminoglycosides

A laboratory mutant of Escherichia coli stably resistant to more than 36,000 U ml-1 of polymyxin B was isolated. The mutant exhibited moderate increases in minimum inhibitory concentration to fluoroquinolones and bacitracin but high levels of cross-resistance to beta-lactams and aminoglycosides. However, it remained susceptible to tetracycline, nalidixic acid and novobiocin. Changes were observed in the outer membrane proteins and lipopolysaccharide profile leading to a decrease in permeability as evident from reduction in the following: (i) minimum inhibitory concentration values in the presence of Tween 80, (ii) uptake of 1-N-phenyl naphthylamine and norfloxacin, (iii) hydrolysis of beta-lactams and (iv) diffusion of lactose and cefazolin into proteoliposomes reconstituted with outer membrane proteins. We therefore suggest that the novel pattern of cross-resistance of our isolate is due to the decrease in its permeability.

Antimicrobial susceptibility of Haemophilus influenzae in the respiratory tracts of patients with cystic fibrosis

We analyzed the antimicrobial susceptibilities of Haemophilus influenzae isolates from 157 sputum specimens prospectively collected from 39 cystic fibrosis (CF) patients during a 2-year study. These isolates were characterized by random amplified polymorphic DNA analysis and major outer membrane protein (MOMP) analysis to identify H. influenzae strains and MOMP variants and to assess their persistence in the respiratory tract. Among the 247 H. influenzae isolates, 16 (6.5%) produced beta-lactamase. The 231 beta-lactamase-negative isolates represented 85 H. influenzae strains, 61 MOMP variants derived from 27 of these strains, and 85 persistent isolates identical to strains or MOMP variants. All beta-lactamase-negative isolates were tested for susceptibility to ampicillin, amoxicillin-clavulanic acid, cefuroxime, cefotaxime, cefaclor, imipenem, tetracycline, and trimethoprim-sulfamethoxazole by disk diffusion testing. Eleven (13%) H. influenzae strains, 18 (30%) MOMP variants, and 30 (35%) persistent isolates were resistant to one or more of the antibiotics tested. Antimicrobial susceptibility was decreased among MOMP variants and persistent isolates compared to nonpersistent H. influenzae strains, and changes in susceptibility occurred irrespective of MOMP variation. We conclude that the decreased antimicrobial susceptibility of H. influenzae during persistence contributes to the poor eradication of H. influenzae from the respiratory tracts of CF patients.

Modulation by zinc of the glutamate transporters in glial cells and cones isolated from the tiger salamander retina

1. Zinc may be released from some presynaptic glutamatergic neurons, including hippocampal mossy fibres and retinal photoreceptors. We whole-cell-clamped glial (Müller) cells isolated from the salamander retina to investigate the effect of zinc on glutamate transporters in these cells. Glutamate-evoked currents in these cells are generated largely by carriers homologous to the mammalian GLAST/EAAT1 transporter. 2. Zinc inhibited both glutamate uptake into the cells, and glutamate release by reversal of the uptake process. The IC50 for inhibition of uptake (< 1 microM) was similar to or below the values for zinc modulating NMDA, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) and GABA receptors, and 100-fold less than the calculated value for the rise in extracellular zinc concentration evoked by depolarization with potassium in area CA3 of the hippocampus. 3. Although zinc altered the apparent affinity of the transporter for glutamate and Na+, it did not act simply by binding competitively to the glutamate-, Na(+)-, K(+)- or H(+)-binding sites on the transporter. Zinc inhibited both forward and reversed glutamate transport from the outside of the cell membrane, but not from the inside. The inhibitory action of zinc on uptake was voltage independent, indicating a zinc-binding site outside the membrane field. 4. As well as inhibiting glutamate transport, zinc potentiated activation of the anion conductance in the Müller cell glutamate transporter. However, zinc reduced the current mediated by the anion conductance in the cone synaptic terminal glutamate transporter (homologous to the mammalian EAAT5), indicating that zinc has different actions on different glutamate transporter subtypes. 5. By acting on glutamate transporters, zinc may have a neuromodulatory role during synaptic transmission and a neuroprotective role during transient ischaemia.

Association of resistance to trimethoprim/sulphamethoxazole, chloramphenicol and quinolones with changes in major outer membrane proteins and lipopolysaccharide in Burkholderia cepacia

Antibiotic treatment options for Burkholderia cepacia infection are limited because of high intrinsic resistance. The problem is complicated by development of cross-resistance between antibiotics of different classes. We isolated antibiotic-resistant mutants by stepwise exposure to chloramphenicol (Chlor) and to trimethoprim/sulphamethoxazole (T/S) for four B. cepacia strains: ATCC13945, Per (clinical isolate), Cas and D4 (environmental isolates). Chlor(r) mutants did not produce chloramphenicol acetyl-transferase. Cross-resistance, defined as greater than four-fold increase in MIC by microtitre dilution method, was consistently seen in both types of mutants. For chloramphenicol-resistant (Chlor[r]) and trimethoprim/sulphamethoxazole-resistant (Tr/Sr) mutants of B. cepacia ATCC13945 and Cas, no MIC change was seen for piperacillin, ceftazidime, rifampicin, gentamicin, tobramycin, polymyxin B or azithromycin. B. cepacia-Per and -D4 mutants showed cross-resistance to ceftazidime and to piperacillin. Comparison of outer membrane protein (OMP) profiles of B. cepacia and their mutants by SDS-PAGE revealed Tr/Sr) mutants to be deficient in a major OMP (molecular weight 39-47 kDa). Tr/Sr mutants also expressed additional OMPs not found in wild type strains at 75-77 kDa for B. cepacia-ATCC13945 and -Cas, and 20-21 kDa in B. cepacia-D4 and -Per. No OMP changes occurred in Chlor(r) mutants. Lipopolysaccharide (LPS) profiles of each type of mutant showed new high and low molecular weight LPS bands. Cross-resistance seems to be mediated by alterations in porin and LPS for Tr/Sr mutants, but only by LPS in Chlor(r) mutants.

Carbapenem resistance in a clinical isolate of Citrobacter freundii

Carbapenem resistance was studied in two sets of Citrobacter freundii strains: (i) strain CFr950, resistant to imipenem (MIC, 16 microg/ml) and isolated in vivo during imipenem therapy, and strain CFr950-Rev, the spontaneous, imipenem-susceptible revertant of CFr950 selected in vitro, and (ii) strains CFr801 and CFr802, two imipenem-resistant mutants selected in vitro from the susceptible clinical isolate CFr800. In all strains, whether they were imipenem-susceptible or -resistant strains, production of the cephalosporinase was derepressed and their Km values for cephaloridine were in the range of 128 to 199 microM. No carbapenemase activity was detected in vitro. The role of cephalosporinase overproduction in the resistance was demonstrated after introduction of the ampD gene which decreased the level of production of cephalosporinase at least 250-fold and resulted in an 8- to 64-fold decrease in the MICs of the carbapenems. The role of reduced permeability in the resistance was suggested by the absence, in CFr950 and CFr802, of two outer membrane proteins (the 42- and 40-kDa putative porins whose levels were considerably decreased in CFr801) and the reappearance of the 42-kDa protein in imipenem-susceptible strain CFr950-Rev. This role was confirmed after introduction of the ompF gene of Escherichia coli into the CFr strains, which resulted in 8- to 16-fold decreases in the MICs of carbapenems for CFr802 and CFr950. We infer from these results that the association of reduced, porin-mediated permeability with high-level cephalosporinase production, observed previously in other gram-negative bacteria, may also confer carbapenem resistance on C. freundii.

Outer membrane proteins of Methylococcus capsulatus (Bath)

Membranes obtained from whole-cell lysates of Methylococcus capsulatus (Bath) were separated by Triton X-100 extraction. The resulting insoluble fraction was enriched in outer membranes as assessed by electron microscopy and by the content of beta-hydroxy palmitic acid and particulate methane monooxygenase. Major proteins with molecular masses of approximately 27, 40, 46, 59, and 66 kDa were detected by SDS-PAGE of the Triton-X-100-insoluble membranes. MopA, MopB, MopC, MopD, and MopE (Methylococcus outer membrane protein) are proposed to designate these proteins. Several of the Mop proteins exhibited heat-modifiable properties in SDS-PAGE and were influenced by the presence of 2-mercaptoethanol in the sample buffer. The 46- and 59-kDa bands migrated as a single high-molecular-mass 95-kDa oligomer under mild denaturing conditions. When reconstituted into black lipid membranes, this oligomer was shown to serve as a channel with an estimated single-channel conductance of 1.4 nS in 1 M KCl.

Postantibiotic effect of norfloxacin and its influence on profiles of outer membrane proteins of Pseudomonas aeruginosa

Norfloxacin at suprainhibitory concentrations induced postantibiotic effects (PAEs) in the range of 6 h to 11.4 h (2.MIC) and of 10.1 to more than 13.4 h (4, MIC) against three P. aeruginosa strains. After PAEs, the outer membrane profile proteins of the strains studied was changed. Overproduction of 41 kDa protein as well as strong reduction of 23 kDa and 45 kDa proteins were found in all three strains. A reduction of the protein band in the range of 35 kDa was also observed in all strains, but to a smaller extent.

Outer membrane proteins and elastase of Pseudomonas aeruginosa after the postantibiotic effect induced by amikacin

Postantibiotic effects were induced in three Pseudomonas aeruginosa strains after short-term treatment (30 min) with amikacin at suprainhibitory concentrations in the range of 0.6-1.2 h (2 x MIC) and of 3.0-4.4 h (4 x MIC). Significant suppression of P. aeruginosa elastase activity was found after treatment with amikacin at 4 x MIC concentration (to 10-69.2% of the control values); 2 x MIC was less effective. The profile of outer membrane proteins of P. aeruginosa was not changed after treatment with amikacin at both concentrations.

gamma-Glutamyltransferase from the outer cell envelope of Treponema denticola ATCC 35405

The human oral spirochete Treponema denticola ATCC 35405 was shown to exhibit relatively high enzyme activity toward the gamma-glutamyl amide bond present in N-gamma-L-glutamyl-4-nitroaniline. The enzyme responsible for this catalysis (gamma-glutamyltransferase [GGT]; EC 2.3.2.2) was purified by means of fast protein liquid chromatography to two sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE)-pure forms from a mild (0.1%) Triton X-100 extract of washed cells. The GGT was studied primarily with regard to its hydrolytic activity by using N-gamma-L-glutamyl-4-nitroaniline as a substrate, although the GGT was shown to catalyze transpeptidation reactions. The high-molecular-mass form of the GGT gave a value of about 213 kDa by SDS-PAGE when heat treatment was omitted and one of 26 kDa after heat treatment; mass spectrometry gave a value of 26.877. The larger form may represent an aggregate with nonprotein structures (possibly of a carbohydrate nature). The preliminary N-terminal sequence of the GGT is MKKPLIGITGSXLYETSQXXF. The enzyme was highly active on glutathione, transferring its Glu residue either to a water molecule or to the Gly-L-Leu dipeptide. The GGT stability was absolutely dependent on the presence of free thiol(s), while no evidence of metalloenzyme nature was obtained. The proposed location of the GGT in the outer cell envelope and its high activity on glutathione, a major nonprotein thiol present in virtually all cells, suggest that the GGT may play a role in the propagation of T. denticola within inflamed periodontal tissues.

Evolution of cell-surface acid phosphatase of Burkholderia pseudomallei

Acid phosphatase active fractions were obtained from cell-free extract, outermembrane fraction and culture filtrate of Burkholderia pseudomallei by column chromatography with sepharose 6B and DEAE cellulose. The comparison of the elution patterns of protein, sugar and enzymatic activity among these three components suggested that the enzyme is a glycoprotein evolving from premature proteins through glycosylation and that the enzyme is translocated during glycosylation from the cytoplasm to the outer membrane and finally excreted into the environment. When tunicamycin, a glycosylation inhibitor, was added to the culture, the peaks of sugar and enzymatic activity were lowered concomitantly leaving the protein peak unchanged in the elution pattern of the culture filtrate. The affinity of the bacterial surface to antienzyme sera was demonstrated by immuno-fluorescence microscopy.

Imipenem resistance among Acinetobacter baumannii: association with reduced expression of a 33-36 kDa outer membrane protein

The mechanism of imipenem resistance of two Acinetobacter baumannii isolates (A-1 and A-24) was characterized in this study. A spontaneous revertant (A-1 (rev)) derived from isolate A-1 showed susceptibility to imipenem. beta-Lactamase hydrolysis studies showed no evidence of an imipenem hydrolyzing enzyme among A-1, A-24, A-1 (rev), or two other imipenem susceptible A. baumannii isolates. Outer membrane protein (OMP) analysis indicated decreased expression of a 33-36 kDa protein by isolates A-1 and A-24 when compared with A-1 (rev) and the other A. baumannii isolates. In conclusion, decreased expression of a 33-36 kDa OMP is associated with imipenem resistance among A. baumannii.

Pro-OmpA derivatives with a His6 tag in their N-terminal "translocation initiation domains" are arrested by Ni2+ at an early post-targeting stage of translocation

We examined in vitro translocation of pro-OmpA derivatives with a His6 tag at various positions in their mature proteins and with a c-Myc tag at their C termini across inverted membrane vesicles of Escherichia coli. Those with a His6 tag in the N-terminal region of the mature domain, which corresponds to the "translocation initiation domain" proposed previously (Andersson, H., and von Heijne, G. (1991) Proc. Natl. Acad. Sci. U. S. A. 88, 9751-9754), could not be translocated in the presence of 100 micron Ni2+, while OmpA derivatives with a His6 tag in the middle of or at the C terminus did not show such Ni2+ sensitivity. The inhibitory action of Ni2+ on pro-3His-OmpA' (with a His6 tag after the third amino acid of the mature OmpA-c-Myc region) translocation was exerted only during early events, after which it became ineffective. The inhibition point of Ni2+ was suggested to lie between membrane targeting and exposure of the signal cleavage site to the periplasm since the unprocessed and membrane-bound form of pro-3His-OmpA' was accumulated by the addition of Ni2+. The Ni(2+)-"trapped" precursor was released from its translocation block by 30 mM histidine, which should compete with the His6 tag on the precursor protein for formation of a Ni2+ chelating complex. We propose that Ni2+ confers a reversible positive charge effect on the His6-tagged initiation domain of the pro-OmpA derivatives and inhibits an early event(s) of protein translocation, such as presentation of the precursor to the membranous part of the translocase. This system will be useful in dissecting early events of the protein translocation pathway.

SurA assists the folding of Escherichia coli outer membrane proteins

Many proteins require enzymatic assistance in order to achieve a functional conformation. One rate-limiting step in protein folding is the cis-trans isomerization of prolyl residues, a reaction catalyzed by prolyl isomerases. SurA, a periplasmic protein of Escherichia coli, has sequence similarity with the prolyl isomerase parvulin. We tested whether SurA was involved in folding periplasmic and outer membrane proteins by using trypsin sensitivity as an assay for protein conformation. We determined that the efficient folding of three outer membrane proteins (OmpA, OmpF, and LamB) requires SurA in vivo, while the folding of four periplasmic proteins was independent of SurA. We conclude that SurA assists in the folding of certain secreted proteins.

Mechanism of lantibiotic-induced pore-formation

Nisin and other lantibiotics have a bacteriocidal effect against Gram-positive bacteria, and also inhibit the outgrowth of bacterial spores. The bacteriocidal effect appears to be due to the formation of pores in the bacterial membrane. In the absence of anionic membrane phospholipids, the lantibiotic nisin acts as an anion selective carrier. In the presence of anionic phospholipids, nisin forms nonselective, transient, multi-state pores in cells, proteoliposomes, liposomes and black lipid membranes. Pore formation involves distinct steps. First, nisin associates tightly with the anionic membrane surface leading to a high local concentration. This results in a disturbance of the lipid dynamics near the phospholipid polar head group-water interface, and an immobilization of lipids. In the presence of a transmembrane electrical potential above the threshold level, the molecules reorient, presumably as an aggregate, from a surface-bound into a membrane-inserted configuration. Co-insertion of bound, anionic phospholipids results in bending of the lipid surface giving rise to a wedge-like, nonspecific, water-filled pore.

Effect of chlorhexidine on the adherence properties of Porphyromonas gingivalis

Chlorhexidine is a bisbiguanide compound that possesses substantive and antibacterial properties. The aim of the present investigation was to evaluate the effect of chlorhexidine on the adherence of Porphyromonas gingivalis to epithelial cells and erythrocytes. Both pretreatment of bacterial cells with chlorhexidine (> 20 micrograms/ml) and incorporation of chlorhexidine in an adherence assay markedly affected the ability of P. gingivalis to adhere to epithelial cells. Chlorhexidine also strongly inhibited hemagglutination by P. gingivalis. It is suggested that chlorhexidine binds to cells, altering the structural conformation of the outer membrane and reducing adherence. This ability of chlorhexidine to prevent bacterial adherence may represent an additional mechanism by which this antimicrobial agent exerts its beneficial clinical effect when used as an adjunct to periodontal therapy.

Influence of pH on adaptive resistance of Pseudomonas aeruginosa to aminoglycosides and their postantibiotic effects

Adaptive resistance to aminoglycosides in Pseudomonas aeruginosa and other gram-negative bacilli is usually induced by the initial exposure to the drug. We investigated the influence of pH on the adaptive resistance of a clinical P. aeruginosa strain to aminoglycosides in vitro and on their postantibiotic effects. For adaptive resistance, the first-exposure concentrations of both amikacin and netilmicin were one, two, four, and eight times the MIC of each drug and the second-exposure concentrations were two times the MIC of each drug. Adaptive resistance was greater and more prolonged with higher initial aminoglycoside concentrations, and the bactericidal effects of the aminoglycosides were concentration dependent at pH 7.4. At pH 6.5, the killing rates of amikacin and netilmicin were far lower than those observed at pH 7.4. At pH 5.5, amikacin and netilmicin exerted practically no bactericidal effect on the P. aeruginosa strain used. However, with media at pH 5.5 and 6.5, adaptive resistance of P. aeruginosa preexposed to amikacin and netilmicin was also clearly exhibited, with the degree of adaptive resistance depending on the bactericidal effects of both drugs on nonpreexposed controls. Maximal adaptive resistance occurred between 0 and 4 h after preexposure. The postantibiotic effects of amikacin and netilmicin against the P. aeruginosa strain were shown to be concentration dependent and were reduced at acidic pHs. No changes in outer and inner membrane proteins occurred during the adaptive-resistance interval.

Alterations in cell pigmentation, protein expression, and photosynthetic capacity of the cyanobacterium Oscillatoria tenuis grown under low iron conditions

To better describe the iron-limited nutrient status of aquatic photosynthetic microorganisms, we examined the effects of iron limitation on pigment content, maximum rates of photosynthetic oxygen evolution, and respiratory oxygen consumption in the filamentous cyanobacterium Oscillatoria tenuis Ag. Within the range of iron (4.2 x 10(-5)-5.1 x 10(-9) M FeCl3), growth rates were not limited by photosynthetic capacity but rather by another, as of yet undetermined, iron-requiring cellular function. We have also investigated membrane proteins that are induced when the cells are grown in low iron medium. Using membrane fractionation techniques we were able to recognize specific proteins localized in the outer membrane and periplasmic space of O. tenuis. The recovery of growth rates at low iron levels occurred in parallel with the induction of these proteins and the production of extracellular siderophores. The additional iron acquired by this high affinity transport system did not reestablish photosynthesis in O. tenuis to the iron-satiated level but did reestablish growth to iron-replete levels. Oscillatoria tenuis appears to invoke an alternate physiology to compensate for iron deficiency.

Pneumococcal polysaccharide-meningococcal outer membrane protein complex conjugate vaccine is immunogenic in infants and children

Sixty-two infants and 31 toddlers were vaccinated with the tetravalent pneumococcal conjugate vaccine PncOMPC consisting of the capsular polysaccharide of pneumococcal types 6B, 14, 19F, and 23F conjugated to the outer membrane protein complex of Neisseria meningitidis. Infants were vaccinated at 2, 4, and 6 months (group A) or at 4, 6, and 14 months (group B); toddlers were vaccinated at 24 or at 24 and 26 months of age. The IgG responses to the four pneumococcal polysaccharide types were measured by EIA. In infants, types 14 and 19F induced a significant response after the first dose and types 6B and 23F after the second dose. A clearcut booster response was seen to the booster dose given at 14 months, indicating immunologic priming by the primary series at 2-6 months of age. The responses of the toddlers to one or two doses of the vaccine were very similar to the responses in infants.

Evidence that His110 of the protein FadL in the outer membrane of Escherichia coli is involved in the binding and uptake of long-chain fatty acids: possible role of this residue in carboxylate binding

The binding of exogenous fatty acids to the outer-membrane protein FadL of Escherichia coli is specific for long-chain fatty acids (C14-C18). Oleoyl alcohol [(Z)-9-octadecen-1-ol] and methyl oleate were unable to displace FadL-specific binding of [3H]oleate (C18:1), suggesting that the carboxylate of the long-chain fatty acid was required for binding. Therefore the binding of exogenous fatty acids to FadL is governed, in part, by the carboxy group of the long-chain fatty acid. Treatment of whole cells with 1 mM diethyl pyrocarbonate (DEPC) depressed binding by 43-73% over the range of oleate concentrations used (10-500 nM). On the basis of these results and the notion that histidine residues often play a role involving proton transfer and charge-pairing, the five histidine residues within FadL (His110, His226, His327, His345 and His418) were replaced by alanine using site-directed mutagenesis. Altered FadL proteins were correctly localized in the outer membrane at wild-type levels and retained the heat-modifiable property characteristic of the wild-type protein. Initial screening of these fadL mutants revealed that the replacement of His110 by Ala resulted in a decreased growth rate on minimal oleate/agar plates. The rates of long-chain fatty acid transport for delta fadL strains harbouring each mutation on a plasmid, with the exception of fadLH110A, were the same, or nearly the same, as those for the wild-type. fadLH110A was also defective in binding, arguing that the functional effect of this mutation was at the level of long-chain-fatty-acid binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Conservation of immune responses to proteins isolated by preparative polyacrylamide gel electrophoresis from the outer membrane of nontypeable Haemophilus influenzae

Outer membrane proteins P2, P4, and P6 and two with molecular masses of 26 and 28 kDa have been purified from a strain of nontypeable Haemophilus influenzae by a preparative form of polyacrylamide gel electrophoresis (PAGE). Outer membrane protein P6, with a molecular mass of 16 kDa (determined by sodium dodecyl sulfate [SDS]-PAGE) was purified by both native PAGE and SDS-PAGE from three strains of nontypeable H. influenzae and one strain of type b H. influenzae. The same conditions were required for purification from each strain. The suitability of proteins isolated by these methods was assessed by studying the immune response of rats immunized with P6 in incomplete Freund's adjuvant into the Peyer's patches. P6 purified by either native PAGE or SDS-PAGE did not differ significantly from P6 purified by gel filtration and anion-exchange chromatography in the ability to enhance pulmonary clearance of live bacteria. This study also investigated the effects of SDS on P2 immunological responses in vivo and the effects of the reagents Zwittergent and sodium lauryl sarcosinate on outer membrane protein lymphocyte-proliferative responses in vitro. It was found that the presence of SDS in the immunization emulsion enhanced the antigen-specific cell-mediated response but suppressed the antigen-specific antibody responses. The presence of residual traces of Zwittergent in an outer membrane protein preparation inhibited antigen-specific cell-mediated proliferation, whereas extraction of outer membrane proteins with sodium lauryl sarcosinate did not inhibit antigen-specific proliferation. These results demonstrate that preparative PAGE is a suitable method for the purification of proteins from the outer membrane of H. influenzae required for investigation of their immunological significance as vaccine candidates and that traces of reagents used during protein purification may play an important role in determining the success of in vivo and in vitro studies.

Profiles of outer membrane proteins and lipopolysaccharide of Pseudomonas aeruginosa grown in the presence of sub-MICs of macrolide antibiotics and their relation to enhanced serum sensitivity

We have previously reported that erythromycin at sub-inhibitory concentrations enhanced the serum sensitivity of some Pseudomonas aeruginosa strains. To explore the mechanism of this effect, we have now examined the influence of macrolide antibiotics on outer membrane proteins and lipopolysaccharide (LPS) of P. aeruginosa. The strains S-6 and PAO-1 of P. aeruginosa were used as susceptible and resistant strains respectively to assess enhancement of serum sensitivity by erythromycin. The strain S-6 became more serum-sensitive when grown on agar with sub-MICs of erythromycin or azithromycin, but not of josamycin, whereas no change was observed in the serum sensitivity of the strain PAO-1 after growth with any of these antibiotics. The analysis of outer membrane proteins showed that erythromycin treatment resulted in a reduction in the amount of the 38 kDa protein (OprF) and in a prominent increase of 41 kDa protein band in the strain S-6, but not in the strain PAO-1. By an immunoblotting assay, this 41 kDa protein was shown to be highly reactive to the immune serum against untreated P. aeruginosa. LPS of the strain S-6 were examined by SDS acrylamide gel electrophoresis. The treatment with erythromycin or azithromycin, but not with josamycin, reduced the amounts of LPS species with lower molecular weights although the levels of LPS species with high molecular weights were similar to those of untreated bacteria. These results suggest that the enhanced serum sensitivity of P. aeruginosa by erythromycin is associated with changes in bacterial surface components, such as outer membrane proteins and LPS.

Mechanisms of quinolone resistance

Two mechanisms of resistance to fluoroquinolones are known: (i) alteration of the molecular target of quinolone action-DNA gyrase, and (ii) reduction of the quinolone accumulation. Mutations altering the N-terminus of the gyrase A subunit, especially those around residues Ser83 and Asp87, significantly reduce the susceptibilities towards all quinolones, while alterations of the gyrase B subunit are rarely found and are of minor importance. Reduced drug accumulation is associated with alterations of the outer membrane protein profile in gram-negative bacteria. Such mutations include the marA locus in Escherichia coli and result in low level resistance towards quinolones and unrelated drugs. Increased activity of naturally existing efflux systems, such as the transmembrane protein NorA of staphylococci, may also lead to reduced accumulation in gram-positive and gram-negative bacteria. Clinical fluoroquinolone resistance is rarely found in intrinsically highly susceptible organisms such as Enterobacteriaceae and involves a combination of at least two mutations. In contrast, species with moderate intrinsic susceptibility such as Campylobacter jejuni, Pseudomonas aeruginosa, and Staphylococcus aureus require only one mutation to become clinically resistant. As a consequence development of resistance during therapy may result from acquisition of already resistant strains in the case of susceptible species, and selection of mutants in the case of less susceptible species.

Calcium ion-mediated opening of the channel gate in the Pseudomonas aeruginosa porin

The gate-forming domain of protein D2 (OprD2) in the outer membrane of Pseudomonas aeruginosa contains an amino acid sequence homologous to the calcium-binding site of the myosin light chain. This observation lets us to test the effect of Ca2+ on the channel function of OprD2. The diffusion rate of p-nitrophenyl phosphate (PNPP) through OprD2 was 2.3 times higher in the presence of mM order of Ca2+, but not Mg2+ or Mn2+. Concomitantly, the intrinsic fluorescence emission of OprD2 became about 10% lower in the presence of Ca2+. As the proteolytic cleavage of the gate-forming domain of OprD2 results the channel activity about 7 times higher, we tested the effect of Ca2+ on the solute permeability through the trypsin-treated OprD2. The diffusion rate of PNPP in the trypsin-treated OprD2 appeared to be nearly identical in the presence and absence of Ca2+. The result suggests that Ca2+ activates the OprD2 channel exerting its effect on the gate-forming domain.

Outer membrane proteins and lipopolysaccharide changes after exposure of Pseudomonas aeruginosa to antibacterial drugs

Many studies show the low sensitivity of P. aeruginosa to antimicrobial agents, due to the permeability degree of the outer membrane (OM). Since the alterations of porins and lipopolysaccharides (LPS) are responsible for permeability to antibacterial drugs, the aim of this study was to evaluate the sub-M.I.C. effects of different antibiotics on the OM components. The M.I.C.s of five P. aeruginosa strains were determined and sub-M.I.C.s. versus PD-131,628, a new difluoroquinolone, aztreonam and netilmicin were calculated. The OM components were extracted before and after contact with the antibacterial drugs. We noted a decrease in M.I.C. values in two strains, and simultaneously an increase in the 19 and 38 Kd bands after using aztreonam. The M.I.C.s tended to increase in three strains after using netilmicin. The electrophoresis profile showed a decrease in the 38, 41 and 45 Kd bands and in one strain also in the 19 Kd band. The use of the quinolone did not significantly modify the M.I.C. values, although an evident increase in 38 and 41 Kd bands occurred in three strains. LPS alterations were observed with aztreonam and netilmicin, but not when PD 131,628 was used.