Antibiotic function The β-lactam family of antibiotic molecules consists of four groups: cephalosporins, monobactam, penicillins, and<unk> s. Different drugs, such as ertapenem, imipenem, meropenem, and doripenem, belong to the class of<unk>  antibiotics. These antibiotics share common structure and mechanism of action. They enter the periplasmic space through porins, where they then inhibit transpeptidases (which are also known as penicillin-binding proteins (PBPs)), enzymes that facilitate peptide cross-links during cell wall synthesis. Their binding to the PBP active site is facilitated in part by their common structure, which is similar to that of D-alanyl-D-alanine. D-alanyl-D-alanine is a residue on the NAM peptide subunit involved in building peptidoglycan.<unk>  covalently binds to PBPs, which causes transpeptidases to irreversibly lose their catalytic activity. Inhibition of transpeptidases prevents the formation of cross-links between peptidoglycan polymers and causes a build-up of peptidoglycan precursors. Newly formed peptidoglycan is weakened from the absence of cross-linkages. The continued activity of autolysins, that function like lysozymes and cleave glycosidic and peptide bonds of peptidoglycan in periplasm, weakens the cell wall and leads to osmotic bursting of the bacterial cell. A unique quality of<unk> s is their resistance to hydrolysis by bacterial plasmid and chromosomally mediated extended-spectrum β-lactamases (ESBL).<unk>  resistance In general,<unk> , a β-lactam antibiotic, targets cells by inhibiting transpeptidases (penicillin-binding proteins). This prevents synthesis of peptidoglycan, a necessary structural component, leading to cell lysis. Resistance to<unk>  among Gram-negative bacteria in general ( i.e. including bacteria other than<unk>  can be acquired through several mechanisms. # Active transport of<unk>  drugs out of the cell, augmented drug efflux, has been observed in some resistant species. # One mechanism of resistance is mutation in or loss of outer membrane porins, preventing antibiotics from entering the cells. Changes within the porin protein gene cause a frameshift, altering the porin structure and function. Changes in the porin protein hinder the diffusion of<unk>  and other antibiotics into the periplasm. Bacteria that express plasmid-borne extended-spectrum β-lactamases (ESBL) can become<unk> -resistant if an insertion sequence or four-nucleotide duplication is present within chromosomal genes for outer membrane porin proteins. Klebsiella pneumoniae has been associated with the lack of outer membrane porin proteins, OmpK35 and OmpK36. The loss of OmpK36 porins can be attributed to point mutations that result in premature termination of translation, resulting in a truncated and consequently nonfunctional protein. These outer membrane proteins are involved in the transfer of the antimicrobial genetic material in the cell. Loss of either OmpK35 and OmpK36 or only OmpK36 leads to<unk>  resistance. In Klebsiella pneumoniae, the lack of either OmpK35 or OmpK36 leads to<unk>  resistance, but with the lack of both proteins, a high level of resistance is present. An observed 32- to 64-fold increase in minimum inhibitory concentrations occurs for the<unk> s when both proteins are not expressed. #<unk>  produce<unk> ases, a form of β-lactamase. These enzymes cleave the β-lactam ring, an essential component of β-lactam antibiotics that are recognized by and bound to PBPs.<unk> ases are divided into different classes, depending on the structure of the enzyme and the mechanism by which they hydrolyze the β-lactam ring. The two broad categories of<unk> ases are serine-carbopenemases, which contain serine at the active site, and metallocarbapenemases, which contain zinc at the active site. Class A<unk> ases are serine<unk> ases and are encoded on either the chromosome of the bacteria or a plasmid. A serine at position 70 at the active site of this class of enzymes is required for hydrolysis of β-lactams to occur. Class D<unk> ases, also referred to as the OXA β-lactamases, are serine β-lactamases. They are encoded on plasmids and contain a large variability in amino acid sequence. The resistance mechanism for class D<unk> ases is caused by the formation of an acyl intermediate when breaking the β-lactam ring. Class B<unk> ases are metallolactamases and require a zinc at the active site for hydrolysis. # A clinical isolate of E. coli from the sputum sample of a patient admitted to a Beijing hospital was found to acquire resistance to<unk>  through mutations not previously observed. It involved a mutation of a regulator gene marR and the expression of a normally nontranslated membrane porin yedS; both mutations were demonstrated to have effects on the ability of this strain of E.coli to resist<unk> s. The strain lacked the outer membrane proteins OmpF and OmpC, and showed increased expression of a multidrug efflux pump, but did not produce<unk> ase. CR Pseudomonas aeruginosa is commonly present in intensive-care units, and can lead to dangerous infections. In Thai hospitals, of 261 multidrug-resistant samples collected of P. aeruginosa (not part of the<unk> , 71.65% were<unk> -resistant. Site of cleavage by beta-lactamase Transfer between bacteria Gram-negative bacteria can develop and transfer β-lactam resistance (including<unk>  resistance) in many ways. They can generate new extended-spectrum β-lactamases (ESBL) from the existing spectrum of plasmid-mediated β-lactamases through amino acid substitution. They can acquire genes encoding ESBL from environmental bacteria. They can increase the expression of chromosome-encoded β-lactamase genes (bla genes) due to regulatory gene and promoter sequence modifications. They can mobilize bla genes through integrons or horizontal transfer of genomic islands into other Gram-negative species and strains. They can disseminate plasmid-mediated<unk> ases. Finally, they can lower or even inhibit the expression of porin genes. Three major classes of enzymes are involved in<unk>  resistance: class A<unk> ases, class B metallo-β-lactamases (MBL), and class D β-lactamases (OXA). The four known groups of class A<unk> ases are: SME (three types associated with S. marcescens), IMI (present in E. cloacae), GES (16 variants thus far found in P. aeruginosa predominantly but also found in K. pneumoniae and E. coli), and KPC (10 types of K. pneumoniae<unk> ase). At the UVA Medical Center, a transfer mechanism of KPC-dependent<unk>  resistance was discovered in the transmission of a plasmid carrying the transposon (Tn4401), which contains the KPC gene (blaKPC), to several bacteria including Enterobacter cloacae, Klebsiella oxytoca, E. coli, and Citrobacter freundii. The class B metallo-β-lactamases (MBLs) are found largely in Gram-negative bacteria and environmental bacteria. The subclasses of MBL enzymes are B1, B2, and B3. MBLs have diverse enzymatic functions and have the ability to hydrolyze β-lactam antibiotics. The class D β-lactamases (OXA), which hydrolyze oxacillin, provide a good example of the variety of mechanisms that can be used to transfer resistance. The blaOXA genes which encode OXA β-lactamases are found on both chromosomes and plasmids, and they have their natural reservoir in environmental bacteria and deep-sea microflora. Insertions in the vicinity of these genes have been shown to increase the strength of their promoters and increase resistance. Because of these characteristics, a wide geographic dissemination of OXA<unk> ase resistance in particular has occurred. The facilitated spread of<unk>  resistance appears to have multiple origins and repeated introduction into the UK of bacteria with the blaOXA-48 gene via horizontal transfer of similar plasmids to pOXA-48a. A recent study in the UK examined 26 isolates of<unk>  consisting of a diverse set of sequence types (ST) of K. pneumoniae, E. coli, and Enterobacter cloacae producing OXA-48-like<unk> ases. Their findings included: * 25 of the 26 strains had the blaOXA-48 gene. * 21 of these isolates had resistance plasmids that could be transferred by conjugation; 20 of these transformants had the three functional genes, repA, traU, and parA found in pOXA-48a. * In ST38 E. coli, no OXA-48 transconjugants were found and it only had the parA gene. * The Indian strain of K. pneumonia had an OXA-181-encoding plasmid (which had higher resistance to<unk>  and also could not be transferred by conjugation and had none of the three functional genes found in pOXA-48a. Gram-negative bacteria Outer membrane vesicles (OMVs), that can transfer DNA between bacterial cells, are produced by metabolically active bacterial cells, and the OMVs are not the result of cell lysis or cell death. Pathogenic strains can produce about 10-25 times more vesicles than a nonpathogenic strain making this highly relevant to<unk>  resistance transfer. OMVs protect plasmids from being digested extracellularly by nucleases that may be found in the environment, thus favoring horizontal gene transfer.
Carbapenem-resistant enterobacteriaceae
Vancomycin-resistant Enterococcus
Shigatoxigenic and verotoxigenic Escherichia coli
Vancomycin-resistant Staphylococcus aureus
Methicillin-resistant Staphylococcus aureus
Rapid growing mycobacterium
Isolated levocardia
Enteroinvasive Escherichia coli