High-yield expression, refolding, and purification of penicillin-binding protein 2a from methicillin-resistant Staphylococcus aureus strain 27R

Isolation and molecular characterization of antibiotic-resistant lactic acid bacteria from poultry and swine meat products

The transfer via the food chain from animals to humans of microbes that are resistant to antimicrobial agents is of increasing concern. To determine the contributions of nonpathogenic microflora to the occurrence and spread of antibiotic resistance (AR) genes in the food chain, 123 lactic acid bacteria were isolated from 29 samples of raw and processed pork and chicken meat products that had previously tested positive for one or more AR genes that encode clinically relevant ARs: tet(M), tet(O), tet(K), erm(A), erm(B), erm(C), aac (6')-Ie aph (2")-Ia, mecA, and blaZ. All of the isolates were initially tested for their AR gene profiles by PCR. The 59 isolates carrying a tet, erm, or blaZ gene were taken through molecular identification, analyzed by determination of the MIC, and subjected to genetic fingerprinting. Lactococcus garvieae was the predominant species (28 isolates), followed by Lactobacillus plantarum (11 isolates) and L. salivarius (6 isolates), whereas Lactococcus lactis subsp. lactis, Lactobacillus johnsonii, L. reuteri, L. crispatus, and L. brevis were identified at lower frequencies. The tet(M) and erm(B) genes were the most frequently detected. Assessment of multiple resistances in 18 tet positive (tet+) isolates revealed that tet(M) plus erm(B) and tet(K) plus erm(B) were the most frequent AR gene patterns. Partial sequencing of the tet(M) open reading frame of three selected strains showed high sequence similarities (> 99%) with tet(M) genes previously found in human pathogens (Listeria monocytogenes and Neisseria meningitidis). Southern hybridization with plasmid profiles revealed these strains contained tet(M)-carrying plasmids.

Direct detection of antibiotic resistance genes in specimens of chicken and pork meat

Antibiotic resistance (AR) in bacteria, a major threat to human health, has emerged in the last few decades as a consequence of the selective pressure exerted by the widespread use of antibiotics in medicine, agriculture and veterinary practice and as growth promoters in animal husbandry. The frequency of 11 genes [tet(M), tet(O), tet(K), erm(A), erm(B), erm(C), vanA, vanB, aac (6')-Ie aph (2'')-Ia, mecA, blaZ] encoding resistance to some antibiotics widely used in clinical practice was analysed in raw pork and chicken meat and in fermented sausages as well as in faecal samples from the relevant farm animals using a molecular approach based on PCR amplification of bacterial DNA directly extracted from specimens. Some of the 11 AR genes were highly prevalent, the largest number being detected in chicken meat and pig faeces. The genes found most frequently in meat were tet(K) and erm(B); vanB and mecA were the least represented. All 11 determinants were detected in faecal samples except mecA, which was found only in chicken faeces. erm(B) and erm(C) were detected in all faecal samples. The frequency of AR genes was not appreciably different in meat compared to faecal specimens of the relevant animal except for vanB, which was more prevalent in faeces. Our findings suggest that AR genes are highly prevalent in food-associated bacteria and that AR contamination is likely related to breeding rather than processing techniques. Finally, the cultivation-independent molecular method used in this work to determine the prevalence of AR genes in foods proved to be a rapid and reliable alternative to traditional tools.

Reaction of soluble penicillin-binding protein 2a of methicillin-resistant Staphylococcus aureus with beta-lactams and acyclic substrates: kinetics in homogeneous solution

The kinetics of reaction of solubilized penicillin-binding protein 2a (sPBP2a) of methicillin-resistant Staphylococcus aureus with a variety of beta-lactams and acyclic species was studied in homogeneous aqueous solution at 37 degreesC in 25 mM Hepes buffer, pH7.0, containing 1 M NaCl. Under these conditions, but not at lower salt concentrations, protein precipitation did not occur either during or after the reaction. The reactions of beta-lactams in general could be monitored by competition with a chromophoric beta-lactam, nitrocefin, or directly in certain cases by protein fluorescence. Rate constants for reaction of a wide variety of beta-lactams are reported. The interactions are characterized by a slow second-order acylation reaction followed by a slower deacylation. For example, the rate constants for benzylpenicillin were 12 M-1.s-1 and 3x10(-5) s-1 respectively. The acylation is slow in comparison with those of normal non-resistant high-molecular-mass penicillin-binding proteins. sPBP2a also seemed to catalyse the slow hydrolysis of a variety of acyclic depsipeptides but not that of a d-Ala-d-Ala peptide. The reactions with certain depsipeptides also led to protein precipitation. These reactions were, however, not affected by prior blockage of the beta-lactam-binding site by benzylpenicillin and thus might take place elsewhere on the enzyme. Two classes of potential transition- state analogue inhibitors, phosphonate monoesters and boronates, seemed to have little effect on the rate of reaction of sPBP2a with nitrocefin and therefore seem to have little affinity for the beta-lactam-binding/D,D-peptidase site.

Soluble penicillin-binding protein 2a: beta-lactam binding and inhibition by non-beta-lactams using a 96-well format

High level methicillin resistance in Staphylococcus aureus is dependent upon the acquisition of the mecA gene encoding penicillin-binding protein 2a (PBP2a). PBP2a is a member of a family of peptidoglycan biosynthetic enzymes involved in assembly of the cell wall in bacteria and is poorly inactivated by beta-lactam antibiotics. We describe a 96-well-filter binding assay using recombinant, soluble PBP2a which allows for kinetic measurement of penicillin binding. The deacylation rate constant for the PBP2a-penicillin G covalent complex was found to be 5.7 +/- 1.0 x 10(-5) s-1 at 30 degrees C (half-life of approximately 200 min). For the PBP2a acylation reaction, the value of K(m) (penicillin G) = 0.5 +/- 0.1 mM and kcat = 1 x 10(-3) s-1, which yields a second-order rate constant (kcat/K(m)) for inactivation of 2.0 M-1 s-1. Using this assay, several non-beta-lactam inhibitors including Cibacron blue have been found which exhibit IC50 values between 10 and 30 microM. The binding affinities of several carbapenems and beta-lactams correlated well between the filter binding assay described in this report and an electrophoretic assay for PBP2a using membranes prepared form methicillin-resistant S. aureus.

High-level expression of soluble protein in Escherichia coli using a His6-tag and maltose-binding-protein double-affinity fusion system

Using the maltose-binding protein (MBP) fusion vector pMAL-c1 from C. V. Maina et al. (1988, Gene 74, 365-373), we have constructed expression vectors which contain a sequence encoding six consecutive histidine residues (His6-tag) at the 3' end of the MBP-encoding malE gene which is followed by either a thrombin-binding site (LVPRGS) or a factor Xa-binding site (IEGR). The benefits of this approach include; (a) high expression levels of soluble MBP fusion proteins (exceeding 2% of the total cellular protein), (b) high-quality purification of proteins under various conditions (high salt, low salt, denaturing, nondenaturing, etc.), and (c) two alternative protease cleavage sites to test for the most efficient cleavage of each fusion protein. We also constructed these MBP-His 6-tag expression vectors with alternative selection markers (Ampr, Kanr) and alternative promoters (tac, T7). Using these constructs, we expressed and purified several proteins of which we present two, penicillin-binding protein PBP2a and UDP-N-acetylmuramate:L-alanine ligase (MurC), and compare their expression level and purity with other expression systems. We also discuss the use of minimal media with supplements versus rich media and cell growth strategies to optimize the protein yield in general and for isotope labeling.