Aminocyclitol resistance in Staphylococcus aureus: presence of plasmids and aminocyclitol-modifying enzymes

New Staphylococcus aureus incompatibility group 1 plasmids encoding penicillinase production and resistance to different antibacterial agents

Eleven Staphylococcus aureus plasmids encoding penicillinase production and resistance to different antibacterial agents were transferred to laboratory recipient strains in mixed-culture transfer and transduction experiments and characterized by restriction endonuclease analysis and incompatibility. The plasmids were differentiated into four types (types A-D) on the basis of their resistance phenotypes and restriction endonuclease patterns. One type encoded resistance to cadmium and arsenate. The second type encoded resistance to cadmium, mercuric compounds and nucleic acid-binding compounds. The third type encoded resistance to cadmium, kanamycin, neomycin and streptomycin while the fourth type encoded resistance to kanamycin, neomycin and ethidium bromide. Plasmids within the same class were structurally related or similar and were different from those in the other classes. Three plasmids, pWBG626, pWBG628, and pWBG663, representing three of the four plasmid types, belonged to incompatibility group 1. These new plasmids add to the number of known incompatibility group 1 plasmids and have resistance phenotypes which should be useful for studying incompatibility of new S. aureus plasmids.

Detection of genes encoding aminoglycoside-modifying enzymes in staphylococci by polymerase chain reaction and dot blot hybridization

Dot blot hybridization and the polymerase chain reaction (PCR) were used to study aminoglycoside-modifying enzymes in aminoglycoside-resistant staphylococci isolated in hospitals in Kuwait. DNA encoding the acetyltransferase (AAC) (6')-phosphotransferase (APH) (2"), nucleotidyltransferase (ANT) (4') and APH (3') enzymes were detected in Staphylococcus aureus and coagulase negative staphylococci. ANT (4') was the most common enzyme detected. The majority of isolates contained genes for all three modifying enzymes, AAC (6')-APH (2"), ANT (4') and APH (3'); only few isolates carried genes for a single modifying enzyme. Genes encoding the AAC (6')-APH (2") were detected in all except two gentamicin-resistant isolates. In these isolates the genes for the AAC (6')-APH (2") enzyme could not be detected by PCR and dot blot hybridization. Whereas antibiotic resistance testing could be used to predict the presence of the AAC (6')-APH (2") enzyme it was not useful in predicting the presence of the ANT (4') or APH (3') enzymes in gentamicin-resistant isolates. Results obtained with dot blot hybridization were comparable to those obtained with PCR. However, PCR was fast and results were obtained within the same day. Therefore PCR would be preferred for the detection and confirmation of the presence of aminoglycoside-modifying enzymes in clinical microbiology laboratories.

Antimicrobial resistance in staphylococci. Epidemiology, molecular mechanisms, and clinical relevance

Staphylococcal infections continue to pose important clinical problems in children and adults. Antibiotic resistance among the staphylococci has rendered therapy of these infections a therapeutic challenge. Despite early, uniform susceptibility to penicillin, staphylococci acquired a gene elaborating beta-lactamase that rendered penicillin inactive and that is borne by nearly all clinical isolates. "Penicillinase-resistant beta-lactams," such as methicillin, were introduced in the early 1960s, but resistance to them has become an increasing concern. The mechanism of the so-called "methicillin resistance" is complex. Moreover, once confined to the ecology of hospitals and other institutions, a recent increase in community-acquired methicillin-resistant S. aureus infections has been observed. Glycopeptides, until now the only uniformly reliable therapeutic modality, have been increasingly used for therapy of staphylococcal infections. The recent recognition of clinical isolates with reduced susceptibility to glycopeptides is of concern.

Antimicrobial resistance in staphylococci

Staphylococci have developed a variety of strategies for dealing with the presence of antibiotics encountered in clinical environments. Resistance to beta-lactams and other antimicrobial agents has been accomplished by a diverse array of molecular mechanisms. Options available to treat infections caused by staphylococci resistant to methicillin are limited, and the next generation of antibiotics to be introduced, should glycopeptide resistance become an important clinical problem, is not yet on the horizon.

Comparative sensitivity to antibiotics and biocides of methicillin-resistant Staphylococcus aureus strains isolated from Saudi Arabia and Great Britain

Methicillin-resistant Staphylococcus aureus (MRSA) isolated in Saudi Arabia and Great Britain were examined for susceptibility to antibiotics and biocides. The strains differed in their sensitivity patterns. None of the Saudi strains showed resistance to propamidine isethionate, but most of the British gentamicin methicillin-resistant Staph. aureus (GMRSA) strains were highly resistant to this compound and to some other nucleic acid-binding (NAB) compounds. Both groups showed a low level of resistance towards quaternary ammonium compounds (QACs), but resistance to these compounds was not associated with resistance to gentamicin in the Saudi strains. The aminoglycoside-resistant determinants were non-conjugative in these strains. Natural MRSA strains were good recipients for pWG613, but transferred this plasmid in reciprocal crosses at significantly lower rates.

4',4'' adenyltransferase activity on conjugative plasmids isolated from Staphylococcus aureus is encoded on an integrated copy of pUB110

In staphylococci, linked resistance to the aminoglycosides kanamycin, neomycin, paromomycin, and tobramycin (KmNmPmTmr) is generally mediated by an aadD determinant which encodes production of an adenyltransferase aminoglycoside modifying enzyme, AAD(4',4''). The aadD resistance determinant is located on small multicopy plasmids such as pUB110, and has also been found on large multiresistance plasmids and on the chromosome in some strains. Examination of two conjugative plasmids from strains of Staphylococcus aureus isolated in North America indicated that the aadD determinant on these plasmids is located on an integrated copy of pUB110. The integrated pUB110 is flanked by direct repeats of the staphylococcal insertion sequence IS257. Analysis of the conjugative plasmid pSK41 showed an 8-bp duplication of the pUB110 sequence immediately adjacent to flanking IS257 elements, suggesting that integration of pUB110 was mediated by IS257.

Molecular analysis of a gentamicin resistance transposonlike element on plasmids isolated from North American Staphylococcus aureus strains

Plasmid-encoded resistance to the aminoglycosides gentamicin (Gm), tobramycin (Tm), and kanamycin (Km) (GmTmKmr) in strains of Staphylococcus aureus isolated in Australia and North America appears to be mediated by one resistance determinant. In Australian isolates, this determinant is flanked by inverted copies of a 1.3-kb insertion sequence, IS256, thereby forming a composite transposon, Tn4001. Analysis of two conjugative plasmids and a related nonconjugative plasmid from strains of S. aureus isolated in North America showed that the GmTmKmr determinant on these plasmids is also flanked by inverted repeats. In the nonconjugative plasmid, these repeats include only 425 bp of IS256 immediately adjacent to the GmTmKmr region and identical to that on Tn4001. This truncated Tn4001 element is flanked by copies of the insertion element IS257, and together these elements form a truncated Tn4001-IS257 hybrid transposonlike structure. A third copy of IS257 was located 418 bp from the hybrid structure. The truncated Tn4001 and three repeats of IS257 were present at a conserved site on the plasmids studied. Four additional copies of IS257 were identified on the two conjugative plasmids. These elements flank determinants for resistance to the aminoglycosides neomycin and paromomycin and to ethidium bromide and quaternary ammonium compounds, as well as the region involved in conjugative plasmid transfer.

Nucleotide sequence analysis of IS256 from the Staphylococcus aureus gentamicin-tobramycin-kanamycin-resistance transposon Tn4001

Resistance to the aminoglycosides gentamicin, tobramycin and kanamycin (GmTmKmR) in Australian clinical strains of Staphylococcus aureus is commonly carried on the composite transposon Tn4001. The resistance gene aacA-aphD of Tn4001, which encodes a bifunctional AAC(6')-APH(2") modifying enzyme, is flanked by two 1324-bp inverted repeats, IS256L and IS256R, that are identical in sequence. Analysis of the IS256 sequence revealed structural features characteristic of IS elements including 26-bp imperfect terminal inverted repeats and a single open reading frame with coding capacity for a 45.6 kDa protein. The nucleotide sequence of IS256 described here, together with the sequence of the aacA-aphD gene reported previously [Rouch et al., J. Gen. Microbiol. 133 (1987) 3039-3052], completes the entire sequence of Tn4001, which totals 4566 bp.

Mobility of gentamicin resistance genes from staphylococci isolated in the United States: identification of Tn4031, a gentamicin resistance transposon from Staphylococcus epidermidis

Homologous genes encoding resistance to gentamicin, tobramycin, and kanamycin through the bifunctional acetylating [AAC(6')] and phosphorylating [APH(2")] aminoglycoside-modifying enzyme were identified in staphylococci isolated from patients in the United States. The mobility of gentamicin resistance (Gmr) genes found on a prototype conjugative plasmid (pGO1) was compared with that of genes cloned from chromosomal sites. Plasmid-encoded Gmr genes and flanking sequences were introduced onto a temperature-sensitive plasmid (pRN3208) from pGO1 by homologous recombination between insertion sequence-like elements present on both replicons. Growth of Staphylococcus aureus strains containing the temperature-sensitive recombinant (pGO161) at the nonpermissive temperature for plasmid replication (42 degrees C) revealed no translocation of Gmr from its plasmid location. A transposon (Tn551) resident on the same replicon did translocate. Chromosomal Gmr determinants were cloned, together with the gene for trimethoprim resistance (dfrA), from three geographically distinct S. epidermidis isolates; two were subcloned onto temperature-sensitive Escherichia coli-S. aureus shuttle plasmids as 7.2-kilobase BglII fragments. Growth of both recombination-deficient and-proficient S. aureus strains containing the cloned genes at 42 degrees C allowed detection of transposition of Gmr sequences and identification of insertion into random chromosomal sites. We have designated this 5-kilobase transposon from S. epidermidis as Tn4031.

Plasmid fingerprinting of methicillin-resistant Staphylococcus aureus strains isolated in Hamburg

By means of restriction endonuclease digests and DNA/hybridisation studies we analysed ten representative methicillin-resistant Staphylococcus aureus strains of our collection for plasmid similarities and plasmid associated resistance determinants. We found that strains isolated at our laboratory contained identical or at least most similar plasmids. Isolates from another geographical origin showed different plasmid patterns. We found resistance determinants for gentamicin to be chromosomally encoded, whereas resistance to heavy metal ions and chloramphenicol was always plasmid associated. Resistance to trimethoprim, tetracycline and erythromycin was usually chromosomally mediated but could also reside on a plasmid. Our results indicate that methicillin-resistant strains from our collection may have a common origin. The clinical relevance of these results is discussed.

Antimicrobial resistance of Staphylococcus aureus: genetic basis

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Determination of aminoglycoside resistance in Staphylococcus aureus by DNA hybridization

A method is described for identification of the genes conferring aminoglycoside resistance in Staphylococcus aureus by dot-blot and Southern blot techniques. As radioactive probes, fragments of plasmids pAT48, pUBH2, and pH13, carrying the genes for an aminocyclitol-3'-phosphotransferase, an aminocyclitol-4'-adenylyltransferase, and an aminocyclitol-2''-phosphotransferase-aminocyclitol-6'-acetyltransferase, respectively, were used.

Broad geographical distribution of homologous erythromycin, kanamycin, and streptomycin resistance determinants among group D streptococci of human and animal origin

The Emr, Kmr, and Smr determinants of the Streptococcus faecalis R plasmid pJH1 were cloned in Streptococcus sanguis with a streptococcal plasmid vector, pVA380-1. Each cloned determinant was used as a probe in hybridization reactions with dot blots containing plasmid-enriched DNA from 91 group D streptococcal isolates resistant to erythromycin, kanamycin, and streptomycin; the isolates were obtained from animal and human sources in a variety of geographical locations. Nearly 70% of the strains contained DNA that hybridized to each of the three resistance determinants from pJH1. Five plasmids mediating resistance to erythromycin, kanamycin, and streptomycin were examined in more detail. These plasmids varied in size between 26 and 105 kilobase pairs (kbp) and exhibited very different EcoRI restriction patterns. However, each plasmid contained the resistance determinants on a single 13- to 20-kbp EcoRI fragment. Southern blot hybridizations and additional restriction endonuclease digests revealed extensive DNA sequence homology and virtually indistinguishable restriction endonuclease maps within a 9- to 11-kbp region of each plasmid which included the resistance determinants.

Molecular cloning and analysis of Staphylococcus aureus chromosomal aminoglycoside resistance genes

Most of the aminoglycoside resistant Staphylococcus aureus strains isolated in France are resistant to all the antibiotics belonging to this family. Two aminoglycoside-modifying enzymes were detected in the wild-type strains studied: an APH3'III and an AAC6'-APH2". These strains also carry two types of streptomycin resistance: high-level resistance due to chromosomal mutation(s) affecting ribosome affinity and low-level resistance, the mechanism of which was not characterized. All the aminoglycoside resistance genes were located on the chromosome. DNA fragments of 1.5 and 1.95 kb carrying the aphA and aacA genes, respectively, were isolated, by cloning, from the cellular DNA of a clinical isolate. When these genes were introduced into Escherichia coli and Bacillus subtilis strains, the enzymes synthesized were indistinguishable from those produced by the S. aureus strains. When the cellular DNAs of wild-type and resistant strains were hybridized with the cloned fragments, sequences homologous to the fragment carrying the aphA gene were found to be located at the same chromosomal site, while those hybridizing with the fragment carrying the aacA gene were at different chromosomal sites.

Nosocomial infections with methicillin and tobramycin resistant Staphylococcus aureus--implication of physiotherapy in hospital-wide dissemination

From January 1979 to July 1980, methicillin and tobramycin resistant Staphylococcus aureus was isolated from 45 patients at a Veteran's Administration hospital. Of these 45, 36 (80%) had clinical infections involving non-surgical wounds (16), urinary tract (10), blood (8), surgical wounds (7), and sputum (5). The epidemic strain had a single phage type (47/54/75/77/83A), a single, 16.4 Md plasmid, and produced an aminoglycoside inactivating enzyme (AAD[4']). Compared to a control group of patients with nosocomial antibiotic susceptible S. aureus infections, patients with the epidemic strain had more hospital days before a positive culture (p = .004), more severe underlying disease (p = .006), received antibiotics more often (p = .0018), and underwent physiotherapy more often (p = .00007). Although selected environmental and personnel cultures were negative for the epidemic strain, epidemiologic investigation suggested that hospital-wide dissemination of S. aureus may have occurred through patient contacts in the physiotherapy department with subsequent clusters of cases occurring on several wards.

Isolation and preliminary characterization of a plasmid mutant derepressed for conjugal transfer in Staphylococcus aureus

The plasmid pCRG1600 is a 52.9-kb self-transmissible plasmid coding for resistance to aminoglycoside and beta-lactam antibiotics in Staphylococcus aureus. When transferred by transduction, plasmid deletion mutants affecting one or more antibiotic-resistance genes were readily obtained. Of these, one derivative (pCRG1690) was found to exhibit a conjugal transfer frequency ca. 100-fold higher than that of the wild-type plasmid. A preliminary physical-genetic map of pCRG1600 located tra in a 14.6-kb region within the 16.9-kb XbaI-A fragment. An 8.5-kb deletion to the left of tra in pCRG1690 was specifically associated with the increased conjugal transferability of the plasmid. Thus, pCRG1690 appears similar to plasmids derepressed for conjugal transfer (drd) in gram-negative bacterial species.

Tn4001: a gentamicin and kanamycin resistance transposon in Staphylococcus aureus

We describe a 4.5 kilobase transposon, Tn4001, which mediates resistance to gentamicin, tobramycin and kanamycin in Staphylococcus aureus. Originally detected in plasmid pSK1, Tn4001 was shown to undergo rec-independent transposition to the chromosome from this plasmid and from an inserted derivative of the plasmid pII147. Heteroduplexes between plasmids with and without Tn4001 demonstrated a characteristic stem and loop structure with inverted repeats of approx. 1.3 kilobases.

Characterization of plasmids in aminocyclitol-resistant Staphylococcus aureus: electron microscopic and restriction endonuclease analysis

Plasmid species isolated from aminocyclitol-resistant Staphylococcus aureus have been analyzed by restriction endonuclease digestion and electron microscopy. These plasmids can be divided into two interrelated groups; intergroup variability is due to the gain or loss of defined DNA sequences. Plasmids pSJ1 and pSJ24 are related to staphylococcal penicillinase plasmid pI524 which was first described over 20 years ago. Both pSJ1 and pSJ24 differ from pI524 by the acquisition of 8 and 4 kbp, respectively, and encode additional resistance to the antibiotics erythromycin and kanamycin. The gain of these resistance determinants suggests that the evolution of staphylococcal resistance plasmids parallels that observed for plasmids of gram-negative bacteria and has serious implications for the spread of antibiotic resistance among the staphylococci.