An IS257-derived hybrid promoter directs transcription of a tetA(K) tetracycline resistance gene in the Staphylococcus aureus chromosomal mec region

The latent cis-regulatory potential of mobile DNA in Escherichia coli

Transposable elements can alter gene regulation in their host genome, either when they integrate into a genome, or when they accrue mutations after integration. However, the extent to which transposons can alter gene expression, as well as the necessary mutational steps, are not well characterized. Here we study the gene regulatory potential of the prominent IS3 family of transposable elements in E.coli. We started with 10 sequences from the ends of 5 IS3 sequences, created 18,537 random mutations in them, and measured their promoter activity using a massively parallel reporter assay. All 10 sequences could evolve de-novo promoter activity from single point mutations. De-novo promoters mostly emerge from existing proto-promoter sequences when mutations create new -10 boxes downstream of preexisting -35 boxes. The ends of IS3s harbor ~1.5 times as many such proto-promoter sequences than the E. coli genome. We also estimate that at least 26% of the 706 characterized IS3s already encode promoters. Our study shows that transposable elements can have a high latent cis-regulatory potential. Our observations can help to explain why mobile DNA may persist in prokaryotic genomes. They also underline the potential use of transposable elements as a substrate for evolving new gene expression.

'To be, or not to be' - the dilemma of 'silent' antimicrobial resistance genes in bacteria

Antimicrobial resistance is a serious threat to public health that dramatically undermines our ability to treat bacterial infections. Microorganisms exhibit resistance to different drug classes by acquiring resistance determinants through multiple mechanisms including horizontal gene transfer. The presence of drug resistance genotypes is mostly associated with corresponding phenotypic resistance against the particular antibiotic. However, bacterial communities harboring silent antimicrobial resistance genes - genes whose presence is not associated with a corresponding resistant phenotype, do exist. Under suitable conditions, the expression pattern of such genes often revert and regain resistance, and could potentially lead to therapeutic failure. We often miss the presence of silent genes, since the current experimental paradigms are focused on resistant strains. Therefore, the knowledge on the prevalence, importance, and mechanism of silent antibiotic resistance genes in bacterial pathogens is very limited. Silent genes, therefore, provide an additional level of complexity in the war against drug-resistant bacteria, reminding us that not only phenotypically resistant strains but also susceptible strains should be carefully investigated. In this review, we discuss the presence of silent antimicrobial resistance genes in bacteria, their relevance, and their importance in public health.

How Do Transposable Elements Activate Expression of Transcriptionally Silent Antibiotic Resistance Genes?

The rapidly emerging phenomenon of antibiotic resistance threatens to substantially reduce the efficacy of available antibacterial therapies. Dissemination of resistance, even between phylogenetically distant bacterial species, is mediated mainly by mobile genetic elements, considered to be natural vectors of horizontal gene transfer. Transposable elements (TEs) play a major role in this process-due to their highly recombinogenic nature they can mobilize adjacent genes and can introduce them into the pool of mobile DNA. Studies investigating this phenomenon usually focus on the genetic load of transposons and the molecular basis of their mobility. However, genes introduced into evolutionarily distant hosts are not necessarily expressed. As a result, bacterial genomes contain a reservoir of transcriptionally silent genetic information that can be activated by various transposon-related recombination events. The TEs themselves along with processes associated with their transposition can introduce promoters into random genomic locations. Thus, similarly to integrons, they have the potential to convert dormant genes into fully functional antibiotic resistance determinants. In this review, we describe the genetic basis of such events and by extension the mechanisms promoting the emergence of new drug-resistant bacterial strains.

Integron Functionality and Genome Innovation: An Update on the Subtle and Smart Strategy of Integrase and Gene Cassette Expression Regulation

Integrons are considered hot spots for bacterial evolution, since these platforms allow one-step genomic innovation by capturing and expressing genes that provide advantageous novelties, such as antibiotic resistance. The acquisition and shuffling of gene cassettes featured by integrons enable the population to rapidly respond to changing selective pressures. However, in order to avoid deleterious effects and fitness burden, the integron activity must be tightly controlled, which happens in an elegant and elaborate fashion, as discussed in detail in the present review. Here, we aimed to provide an up-to-date overview of the complex regulatory networks that permeate the expression and functionality of integrons at both transcriptional and translational levels. It was possible to compile strong shreds of evidence clearly proving that these versatile platforms include functions other than acquiring and expressing gene cassettes. The well-balanced mechanism of integron expression is intricately related with environmental signals, host cell physiology, fitness, and survival, ultimately leading to adaptation on the demand.

The IS6 family, a clinically important group of insertion sequences including IS26

The IS6 family of bacterial and archaeal insertion sequences, first identified in the early 1980s, has proved to be instrumental in the rearrangement and spread of multiple antibiotic resistance. Two IS, IS26 (found in many enterobacterial clinical isolates as components of both chromosome and plasmids) and IS257 (identified in the plasmids and chromosomes of gram-positive bacteria), have received particular attention for their clinical impact. Although few biochemical data are available concerning the transposition mechanism of these elements, genetic studies have provided some interesting observations suggesting that members of the family might transpose using an unexpected mechanism. In this review, we present an overview of the family, the distribution and phylogenetic relationships of its members, their impact on their host genomes and analyse available data concerning the particular transposition pathways they may use. We also provide a mechanistic model that explains the recent observations on one of the IS6 family transposition pathways: targeted cointegrate formation between replicons.

An analysis of the IS6/IS26 family of insertion sequences: is it a single family?

The relationships within a curated set of 112 insertion sequences (ISs) currently assigned to the IS6 family, here re-named the IS6/IS26 family, in the ISFinder database were examined. The encoded DDE transposases include a helix-helix-turn-helix (H-HTH) potential DNA binding domain N-terminal to the catalytic (DDE) domain, but 10 from Clostridia include one or two additional N-terminal domains. The transposase phylogeny clearly separated 75 derived from bacteria from 37 from archaea. The longer bacterial transposases also clustered separately. The 65 shorter bacterial transposases, including Tnp26 from IS26, formed six clades but share significant conservation in the H-HTH domain and in a short extension at the N-terminus, and several amino acids in the catalytic domain are completely or highly conserved. At the outer ends of these ISs, 14 bp were strongly conserved as terminal inverted repeats (TIRs) with the first two bases (GG) and the seventh base (G) present in all except one IS. The longer bacterial transposases are only distantly related to the short bacterial transposases, with only some amino acids conserved. The TIR consensus was longer and only one IS started with GG. The 37 archaeal transposases are only distantly related to either the short or the long bacterial transposases and different residues were conserved. Their TIRs are loosely related to the bacterial TIR consensus but are longer and many do not begin with GG. As they do not fit well with most bacterial ISs, the inclusion of the archaeal ISs and the longer bacterial ISs in the IS6/IS26 family is not appropriate.

Staphylococcal Plasmids, Transposable and Integrative Elements

Strains of Staphylococcus aureus, and to a lesser extent other staphylococcal species, are a significant cause of morbidity and mortality. An important factor in the notoriety of these organisms stems from their frequent resistance to many antimicrobial agents used for chemotherapy. This review catalogues the variety of mobile genetic elements that have been identified in staphylococci, with a primary focus on those associated with the recruitment and spread of antimicrobial resistance genes. These include plasmids, transposable elements such as insertion sequences and transposons, and integrative elements including ICE and SCC elements. In concert, these diverse entities facilitate the intra- and inter-cellular gene mobility that enables horizontal genetic exchange, and have also been found to play additional roles in modulating gene expression and genome rearrangement.

The Role of ISCR1-Borne POUT Promoters in the Expression of Antibiotic Resistance Genes

The ISCR1 (Insertion sequence Common Region) element is the most widespread member of the ISCR family, and is frequently present within γ-proteobacteria that occur in clinical settings. ISCR1 is always associated with the 3'Conserved Segment (3'CS) of class 1 integrons. ISCR1 contains outward-oriented promoters P_OUT, that may contribute to the expression of downstream genes. In ISCR1, there are two P_OUT promoters named P_CR1-1 and P_CR1-2. We performed an in silico analysis of all publically available ISCR1 sequences and identified numerous downstream genes that mainly encode antibiotic resistance genes and that are oriented in the same direction as the P_OUT promoters. Here, we showed that both P_CR1-1 and P_CR1-2 significantly increase the expression of the downstream genes bla _CTX-M-9 and dfrA19. Our data highlight the role of ISCR1 in the expression of antibiotic resistance genes, which may explain why ISCR1 is so frequent in clinical settings.

Replication of Staphylococcal Resistance Plasmids

The currently widespread and increasing prevalence of resistant bacterial pathogens is a significant medical problem. In clinical strains of staphylococci, the genetic determinants that confer resistance to antimicrobial agents are often located on mobile elements, such as plasmids. Many of these resistance plasmids are capable of horizontal transmission to other bacteria in their surroundings, allowing extraordinarily rapid adaptation of bacterial populations. Once the resistance plasmids have been spread, they are often perpetually maintained in the new host, even in the absence of selective pressure. Plasmid persistence is accomplished by plasmid-encoded genetic systems that ensure efficient replication and segregational stability during cell division. Staphylococcal plasmids utilize proteins of evolutionarily diverse families to initiate replication from the plasmid origin of replication. Several distinctive plasmid copy number control mechanisms have been studied in detail and these appear conserved within plasmid classes. The initiators utilize various strategies and serve a multifunctional role in (i) recognition and processing of the cognate replication origin to an initiation active form and (ii) recruitment of host-encoded replication proteins that facilitate replisome assembly. Understanding the detailed molecular mechanisms that underpin plasmid replication may lead to novel approaches that could be used to reverse or slow the development of resistance.

The impact of insertion sequences on bacterial genome plasticity and adaptability

Transposable elements (TE), small mobile genetic elements unable to exist independently of the host genome, were initially believed to be exclusively deleterious genomic parasites. However, it is now clear that they play an important role as bacterial mutagenic agents, enabling the host to adapt to new environmental challenges and to colonize new niches. This review focuses on the impact of insertion sequences (IS), arguably the smallest TE, on bacterial genome plasticity and concomitant adaptability of phenotypic traits, including resistance to antibacterial agents, virulence, pathogenicity and catabolism. The direct consequence of IS transposition is the insertion of one DNA sequence into another. This event can result in gene inactivation as well as in modulation of neighbouring gene expression. The latter is usually mediated by de-repression or by the introduction of a complete or partial promoter located within the element. Furthermore, transcription and transposition of IS are affected by host factors and in some cases by environmental signals offering the host an adaptive strategy and promoting genetic variability to withstand the environmental challenges.

Copresence of tet(K) and tet(M) in Livestock-Associated Methicillin-Resistant Staphylococcus aureus Clonal Complex 398 Is Associated with Increased Fitness during Exposure to Sublethal Concentrations of Tetracycline

The tetracycline resistance gene tet(K) was shown to be integrated within the predominant staphylococcal cassette chromosome mec (SCCmec) element of Danish livestock-associated methicillin-resistant Staphylococcus aureus CC398 (LA-MRSA CC398). These LA-MRSA CC398 isolates already possessed tet(M), but the acquisition of tet(K) significantly improved their fitness at sublethal concentrations of tetracycline. Because tet(K) is genetically linked to SCCmec, the use of tetracycline in food animals may have contributed to the successful spread of LA-MRSA CC398.

Everyman's Guide to Bacterial Insertion Sequences

The number and diversity of known prokaryotic insertion sequences (IS) have increased enormously since their discovery in the late 1960s. At present the sequences of more than 4000 different IS have been deposited in the specialized ISfinder database. Over time it has become increasingly apparent that they are important actors in the evolution of their host genomes and are involved in sequestering, transmitting, mutating and activating genes, and in the rearrangement of both plasmids and chromosomes. This review presents an overview of our current understanding of these transposable elements (TE), their organization and their transposition mechanism as well as their distribution and genomic impact. In spite of their diversity, they share only a very limited number of transposition mechanisms which we outline here. Prokaryotic IS are but one example of a variety of diverse TE which are being revealed due to the advent of extensive genome sequencing projects. A major conclusion from sequence comparisons of various TE is that frontiers between the different types are becoming less clear. We detail these receding frontiers between different IS-related TE. Several, more specialized chapters in this volume include additional detailed information concerning a number of these.

In a second section of the review, we provide a detailed description of the expanding variety of IS, which we have divided into families for convenience. Our perception of these families continues to evolve and families emerge regularly as more IS are identified. This section is designed as an aid and a source of information for consultation by interested specialist readers.

Genetic requirements for replication initiation of the staphylococcal multiresistance plasmid pSK41

Replication of staphylococcal multiresistance plasmid pSK41 is initiated by binding of the replication initiator protein (Rep) to the Rep boxes, a series of four direct repeats located centrally within the rep gene. A Staphylococcus aureus strain was engineered to provide Rep in trans, allowing localization of the pSK41 origin of replication (oriV) to a 185 bp segment, which included the Rep boxes and a series of downstream direct repeats. Deletion analysis of individual Rep boxes revealed that all four Rep boxes are required for maximum origin activity, with the deletion of one or more Rep boxes having a significant effect on the proficiency of replication. However, a hierarchy of importance was identified among the Rep boxes, which appears to be mediated by the minor sequence variations that exist between them. DNA binding studies with truncated Rep proteins have enabled the DNA binding domain to be localized to the N-terminal 134 amino acids of the protein.

Correlation of the capsular phenotype in Propionibacterium freudenreichii with the level of expression of gtf, a unique polysaccharide synthase-encoding gene

Many food-grade bacteria produce exopolysaccharides (EPS) that affect the texture of fermented food products and that may be involved in probiotic properties. Propionibacterium freudenreichii is a Gram-positive food-grade bacterium with reported probiotic capabilities that is widely used as starter in Swiss-type cheese. In this study, 68 strains of P. freudenreichii were screened for the beta-glucan capsular phenotype by immunoagglutination with a specific antibody and for the presence of the gtf gene coding for polysaccharide synthase. All strains were positive for PCR amplification with gtf gene-specific primers, but the presence of beta-glucan capsular EPS was detected for only 35% of the strains studied. Disruption of gtf in P. freudenreichii revealed that gtf is a unique gene involved in beta-glucan capsular EPS production in P. freudenreichii. The gtf gene was transferred into and expressed in Lactococcus lactis, in which it conferred an agglutination-positive phenotype. Expression of the gtf gene was measured by performing quantitative reverse transcription-PCR assays with RNA from four capsular and three noncapsular strains. A positive correlation was found between the beta-glucan capsular phenotype and gtf gene expression. Sequencing of the region upstream of the gtf open reading frame revealed the presence of an insertion element (IS element) in this upstream region in the four strains with the beta-glucan capsular phenotype. The role of the IS element in the expression of neighboring genes and its impact on interstrain variability of the P. freudenreichii capsule phenotype remain to be elucidated.

Complete nucleotide sequence and comparative analysis of pPR9, a 41.7-kilobase conjugative staphylococcal multiresistance plasmid conferring high-level mupirocin resistance

We have sequenced the conjugative plasmid pPR9, which carries the ileS2 gene, which had contributed to the dissemination of high-level mupirocin resistance at our institution. The plasmid backbone shows extensive genetic conservation with plasmids belonging to the pSK41/pGO1 family, but comparative analyses have revealed key differences that provide important insights into the evolution of these medically important plasmids and high-level mupirocin resistance in staphylococci and highlight the role of insertion sequence IS257 in these processes.

The Staphylococcus aureus pSK41 plasmid-encoded ArtA protein is a master regulator of plasmid transmission genes and contains a RHH motif used in alternate DNA-binding modes

Plasmids harbored by Staphylococcus aureus are a major contributor to the spread of bacterial multi-drug resistance. Plasmid conjugation and partition are critical to the dissemination and inheritance of such plasmids. Here, we demonstrate that the ArtA protein encoded by the S. aureus multi-resistance plasmid pSK41 is a global transcriptional regulator of pSK41 genes, including those involved in conjugation and segregation. ArtA shows no sequence homology to any structurally characterized DNA-binding protein. To elucidate the mechanism by which it specifically recognizes its DNA site, we obtained the structure of ArtA bound to its cognate operator, ACATGACATG. The structure reveals that ArtA is representative of a new family of ribbon–helix–helix (RHH) DNA-binding proteins that contain extended, N-terminal basic motifs. Strikingly, unlike most well-studied RHH proteins ArtA binds its cognate operators as a dimer. However, we demonstrate that it is also able to recognize an atypical operator site by binding as a dimer-of-dimers and the extended N-terminal regions of ArtA were shown to be essential for this dimer-of-dimer binding mode. Thus, these data indicate that ArtA is a master regulator of genes critical for both horizontal and vertical transmission of pSK41 and that it can recognize DNA utilizing alternate binding modes.

Staphylococcus aureus clinical isolate with high-level methicillin resistance with an lytH mutation caused by IS1182 insertion

We previously reported that deficiency of the lytH gene, whose product is homologous to lytic enzymes, caused the elevation of methicillin resistance in Staphylococcus aureus strain SR17238, a strain of S. aureus with a low level of resistance to methicillin (low-level MRSA) (J. Bacteriol. 179:6294-6301, 1997). In this study, we demonstrated that deficiency of lytH caused the same phenomenon in four other clinical isolates of low-level MRSA, suggesting this deficiency to exist in clinical isolates. We therefore searched the region including lytH in 127 clinical isolates of MRSA by PCR and found one strain, SR17164 (methicillin MIC, 1,600 microg/ml), in which the lytH gene was inactivated by insertion sequence IS1182. lytH::IS1182 was replaced with intact lytH in this strain by integration and excision of the plasmid carrying the lytH region. Recombinants with intact lytH genes showed methicillin MICs of 800 microg/ml, twofold lower than those of the recombinants with lytH::IS1182 and the parent. In addition, S. aureus SR17164, which has a high level of methicillin resistance, had properties similar to those caused by lytH deficiency; that is, the resistance levels of strain SR17164 and lytH-deficient variants from strain SR17238 were not significantly affected by llm inactivation, which greatly lowered resistance levels in most other high-level MRSA strains. These findings suggest that lytH inactivation contributed, to some extent, to the resistance level of S. aureus SR17164. To the best of our knowledge, this strain is the first clinical isolate of MRSA for which the genetic base for high-level resistance has been clarified.

Characterization of a new variant of IS257 that has displaced the capsule genes within bovine isolates of Staphylococcus aureus

Many bovine Staphylococcus aureus isolates from Argentina are nontypeable (NT), i.e., they do not produce serotype 5 or 8 capsular polysaccharides (CPs). Some of these NT strains have a deletion of the cap5(8) gene cluster mediated by a variant of IS257, now designated IScap. IScap showed 93% amino acid identity to S. aureus ORF49 but only 85% identity to IS431 from S. aureus N315 and 88% identity to an IS257-like element from bovine strain RF122. Thirty-six (53%) of 68 bovine isolates, drawn from a previously described S. aureus strain collection, carried some variant of IS257, including IScap. Of these 36 IS+ isolates, 6 were CP5+, 1 was CP8+, and 29 were NT. Forty-four of the 68 isolates were NT, and 24 of these 44 NT isolates (55%) exhibited IScap-mediated deletion of the cap5(8) gene cluster. IScap was not found among 20 human NT S. aureus isolates bearing the cap5HIJK genes, which suggests that IScap-mediated deletion of the capsule locus is restricted to bovine strains of S. aureus. We were unable to identify a precursor strain in which IScap flanked the cap5(8) capsule locus, nor were we able to select for deletion of the cap5(8) locus in vitro. Our results support the hypothesis that deletion of the cap5 locus occurred in the distant past and that the relative abundance of these NT strains may be a result of their ability to persist in subclinical mastitis infection in cows.

Modes and modulations of antibiotic resistance gene expression

Since antibiotic resistance usually affords a gain of function, there is an associated biological cost resulting in a loss of fitness of the bacterial host. Considering that antibiotic resistance is most often only transiently advantageous to bacteria, an efficient and elegant way for them to escape the lethal action of drugs is the alteration of resistance gene expression. It appears that expression of bacterial resistance to antibiotics is frequently regulated, which indicates that modulation of gene expression probably reflects a good compromise between energy saving and adjustment to a rapidly evolving environment. Modulation of gene expression can occur at the transcriptional or translational level following mutations or the movement of mobile genetic elements and may involve induction by the antibiotic. In the latter case, the antibiotic can have a triple activity: as an antibacterial agent, as an inducer of resistance to itself, and as an inducer of the dissemination of resistance determinants. We will review certain mechanisms, all reversible, that bacteria have elaborated to achieve antibiotic resistance by the fine-tuning of the expression of genetic information.

Deletion of pT181-like sequence in an smr-encoding mosaic plasmid harboured by a persistent bovine Staphylococcus warneri strain

OBJECTIVES

The aim was to study the persistence and characteristics of Staphylococcus warneri strains resistant to quaternary ammonium compounds (QACs), including sequencing and analysis of two plasmids proved to carry the smr gene.

METHODS

During a 3.5 year period quarter milk samples were collected on three occasions from all lactating cows in a dairy herd. The samples were screened with regard to QAC-resistant bacteria using a selective medium. Thirty randomly selected QAC-resistant S. warneri were typed by PFGE and subjected to plasmid isolation and analysis followed by gene detection using PCR. Two smr-containing plasmids in S. warneri isolates were sequenced.

RESULTS

All isolates from the initial collection of quarter milk contained smr residing on a 5.8 kb plasmid (pSW174), which contained regions with high similarities to various plasmids, including pT181, pSK108 and pPI-2. The pT181-like sequence was flanked by 148 bp direct repeats, denoted ISLE49, with high similarity to previously reported sequences of approximately 148 bp, including ISLE39 flanking the insertion sequence IS257 in methicillin-resistant Staphylococcus aureus. All isolates from subsequent collections of quarter milk harboured a smaller smr-containing plasmid (pSW49). Sequence analyses revealed pSW49 (3552 bp) to be an in-part deleted version of pSW174 (5767 bp).

CONCLUSIONS

The IS-associated elements found in this study may have a wider role in the integration and excision of DNA sequences in staphylococci than previously reported. The mosaic plasmid structure based on genetic elements of various origins contributes to further knowledge on the flexibility of smr-encoding plasmids.

Staphylococcus aureus multiresistance plasmid pSK41: analysis of the replication region, initiator protein binding and antisense RNA regulation

The vast majority of large staphylococcal plasmids characterized to date appear to possess an evolutionarily common replication system, which has clearly had a major impact on the evolution of antimicrobial resistant staphylococci worldwide. Related systems have also been found in plasmids from other Gram-positive genera, including enterococci, streptococci and bacilli. The 46.4 kb plasmid pSK41 is the prototype of a family of conjugative staphylococcal multiresistance plasmids. The replication region of pSK41 encodes a protein product, Rep, which was shown to be essential for replication; mutations that truncated Rep could be complemented in trans. Rep was found to bind in vitro to four tandem repeat sequences located centrally within the rep coding region. An A + T-rich inverted repeat sequence upstream of rep was required for efficient replication, whereas no sequences downstream of rep were necessary. An antisense countertranscript, RNAI, encoded upstream of rep was identified and transcriptional start points for both RNAI and the rep-mRNA were defined.

Positive and negative cis-acting regulatory sequences control expression of leukotoxin in Actinobacillus actinomycetemcomitans 652

Integration of IS1301 into an AT-rich inverted repeat located upstream of the ltx operon was previously shown to confer a hyperleukotoxic phenotype in Actinobacillus actinomycetemcomitans IS1 (T. He, T. Nishihara, D. R. Demuth, and I. Ishikawa, J. Periodontol. 70:1261-1268, 1999), but the mechanism leading to increased leukotoxin production was not determined. We show that an IS1 ltx promoter::lacZ reporter construct expresses 12-fold higher levels of beta-galactosidase activity than a reporter containing the ltx promoter from A. actinomycetemcomitans 652, suggesting that IS1301 increases transcription of the ltx operon. Examination of the IS1301 sequence identified a potential outwardly directed promoter. However, site-specific mutagenesis of the -35 element of the putative promoter had no effect on the transcriptional activity of the IS1 reporter construct. Furthermore, reverse transcriptase PCR and real-time PCR experiments did not detect a transcript that was initiated within IS1301. These results suggest that increased expression of leukotoxin in strain IS1 does not arise from an outwardly directed IS1301 promoter. To determine how IS1301 alters transcriptional regulation of the ltx operon, cis-acting sequences that regulate leukotoxin expression were identified. The AT-rich sequence that resides downstream from the site of IS1301 insertion was shown to function as a positive cis-acting regulator of leukotoxin expression. This sequence resembles an UP element in its location, AT-rich content, and activity and is homologous to the consensus UP element sequence. In addition, a negative cis-acting sequence was identified upstream from the site of IS1301 insertion, and deletion of this region increased promoter activity by fourfold. Mobility shift experiments showed that this region bound to a protein(s) in extracts from A. actinomycetemcomitans 652. The specific sequences required for this interaction were localized to a 26-nucleotide segment of the ltx promoter that resides 17 bp upstream from the site of IS1301 insertion. Together, these results suggest that positive and negative cis-acting sequences regulate leukotoxin expression and that IS1301 may increase transcription of the ltx operon in A. actinomycetemcomitans IS1 by displacing a negative cis-acting regulator approximately 900 bp upstream from the basal elements of the ltx promoter.

Antagonism between aminoglycosides and beta-lactams in a methicillin-resistant Staphylococcus aureus isolate involves induction of an aminoglycoside-modifying enzyme

We encountered three clinical isolates of methicillin-resistant Staphylococcus aureus which were susceptible to netilmicin and arbekacin in the absence of beta-lactam antibiotics but which were resistant to them in the presence of beta-lactam antibiotics. One of these strains, KU5801, was used to further investigate the antagonism between aminoglycosides and beta-lactam antibiotics. beta-Lactam antibiotics induced bacterial synthesis of aminoglycoside-6'-N-acetyltransferase and 2"-O-phosphotransferase [AAC(6')-APH(2")] in association with decreased antimicrobial activities of aminoglycosides. A 14.4-kb EcoRI fragment that included the genes that control for beta-lactam-inducible aminoglycoside resistance was cloned from a 31-kb conjugative plasmid present in KU5801. Restriction fragment mapping and PCR analysis suggested that a Tn4001-like element containing a gene encoding AAC(6')-APH(2") was located downstream from a truncated blaZ gene. The DNA sequence between blaR1 and a Tn4001-like element was determined. The Tn4001-IS257 hybrid structure was cointegrated into the blaZ gene, and the typical sequences for the termination of transcription were not found between these regions. We deduced that antagonism of aminoglycosides by beta-lactam antibiotics in isolate KU5801 involved transcription of the aac(6')-Ie-aph(2")-Ia gene under the influence of the system regulating penicillinase production.