Characterization of the Tet M determinants in urogenital and respiratory bacteria

High Levels of Intrinsic Tetracycline Resistance in Mycobacterium abscessus Are Conferred by a Tetracycline-Modifying Monooxygenase

Tetracyclines have been one of the most successful classes of antibiotics. However, its extensive use has led to the emergence of widespread drug resistance, resulting in discontinuation of use against several bacterial infections. Prominent resistance mechanisms include drug efflux and the use of ribosome protection proteins. Infrequently, tetracyclines can be inactivated by the TetX class of enzymes, also referred to as tetracycline destructases. Low levels of tolerance to tetracycline in Mycobacterium smegmatis and Mycobacterium tuberculosis have been previously attributed to the WhiB7-dependent TetV/Tap efflux pump. However, Mycobacterium abscessus is ∼500-fold more resistant to tetracycline than M. smegmatis and M. tuberculosis In this report, we show that this high level of resistance to tetracycline and doxycycline in M. abscessus is conferred by a WhiB7-independent tetracycline-inactivating monooxygenase, MabTetX (MAB_1496c). The presence of sublethal doses of tetracycline and doxycycline results in a >200-fold induction of MabTetX, and an isogenic deletion strain is highly sensitive to both antibiotics. Further, purified MabTetX can rapidly monooxygenate both antibiotics. We also demonstrate that expression of MabTetX is repressed by MabTetR_x, by binding to an inverted repeat sequence upstream of MabTetR_x; the presence of either antibiotic relieves this repression. Moreover, anhydrotetracycline (ATc) can effectively inhibit MabTetX activity in vitro and decreases the MICs of both tetracycline and doxycycline in vivo Finally, we show that tigecycline, a glycylcycline tetracycline, not only is a poor substrate of MabTetX but also is incapable of inducing the expression of MabTetX. This is therefore the first demonstration of a tetracycline-inactivating enzyme in mycobacteria. It (i) elucidates the mechanism of tetracycline resistance in M. abscessus, (ii) demonstrates the use of an inhibitor that can potentially reclaim the use of tetracycline and doxycycline, and (iii) identifies two sequential bottlenecks-MabTetX and MabTetR_x-for acquiring resistance to tigecycline, thereby reiterating its use against M. abscessus.

Minocycline and Tigecycline: What Is Their Role in the Treatment of Carbapenem-Resistant Gram-Negative Organisms?

Carbapenem-resistant organisms are increasingly common worldwide, particularly in India and are associated with high mortality rates especially in patients with severe infection such as bacteremia. Existing drugs such as carbapenems and polymyxins have a number of disadvantages, but remain the mainstay of treatment. The tetracycline class of antibiotics was first produced in the 1940s. Minocycline, tetracycline derivative, although licensed for treatment of wide range of infections, has not been considered for treatment of multidrug-resistant organisms until recently and needs further in vivo studies. Tigecycline, a derivative of minocycline, although with certain disadvantages, has been frequently used in the treatment of carbapenem-resistant organisms. In this article, we review the properties of minocycline and tigecycline, the common mechanisms of resistance, and assess their role in the management of carbapenem-resistant organisms.

Random insertion and gene disruption via transposon mutagenesis of Ureaplasma parvum using a mini-transposon plasmid

While transposon mutagenesis has been successfully used for Mycoplasma spp. to disrupt and determine non-essential genes, previous attempts with Ureaplasma spp. have been unsuccessful. Using a polyethylene glycol-transformation enhancing protocol, we were able to transform three separate serovars of Ureaplasma parvum with a Tn4001-based mini-transposon plasmid containing a gentamicin resistance selection marker. Despite the large degree of homology between Ureaplasma parvum and Ureaplasma urealyticum, all attempts to transform the latter in parallel failed, with the exception of a single clinical U. urealyticum isolate. PCR probing and sequencing were used to confirm transposon insertion into the bacterial genome and identify disrupted genes. Transformation of prototype serovar 3 consistently resulted in transfer only of sequence between the mini-transposon inverted repeats, but some strains showed additional sequence transfer. Transposon insertion occurred randomly in the genome resulting in unique disruption of genes UU047, UU390, UU440, UU450, UU520, UU526, UU582 for single clones from a panel of screened clones. An intergenic insertion between genes UU187 and UU188 was also characterised. Two phenotypic alterations were observed in the mutated strains: Disruption of a DEAD-box RNA helicase (UU582) altered growth kinetics, while the U. urealyticum strain lost resistance to serum attack coincident with disruption of gene UUR10_137 and loss of expression of a 41 kDa protein. Transposon mutagenesis was used successfully to insert single copies of a mini-transposon into the genome and disrupt genes leading to phenotypic changes in Ureaplasma parvum strains. This method can now be used to deliver exogenous genes for expression and determine essential genes for Ureaplasma parvum replication in culture and experimental models.

Evidence for the predominance of a single tet(M) gene sequence type in tetracycline-resistant Ureaplasma parvum and Mycoplasma hominis isolates from Tunisian patients

Resistance to tetracyclines in genital mycoplasmas is due mainly to acquisition of the tet(M) determinant, which is frequently associated with conjugative transposon elements of the Tn916/Tn1545 family. The aim of the present work was to evaluate the prevalence of tet(M) in Tunisian isolates and to gain an insight into its origin and evolution. Twenty Ureaplasma parvum, two Ureaplasma urealyticum and 48 Mycoplasma hominis isolates, recovered from Tunisian patients with urogenital and infertility disorders, were evaluated for their resistance to tetracyclines and interrogated by PCR amplification for the presence of tet(M) and int-Tn, the gene encoding the integrase of Tn916/Tn1545-like transposons. The resistance rates to tetracyclines were 22.72 and 25.0 % among U. parvum and M. hominis isolates, respectively, with high-level resistance observed in 11 of the 12 resistant M. hominis isolates. All resistant isolates harboured both tet(M) and int-Tn sequences. Nucleotide sequence analysis of the tet(M) amplicon revealed a unique sequence shared by all tetracycline-resistant clinical isolates of both species. Molecular typing indicated that the tetracycline-resistant U. parvum and M. hominis isolates were not clonal. Taken together, these data indicate that a single tet(M) gene sequence type, most probably transmitted via a Tn916/Tn1545-like transposon, contributes to most of the tetracycline resistance in U. parvum and M. hominis isolates in Tunisia. Because this tet(M) gene sequence type was harboured by different Mycoplasma spp. and by phylogenetically distinct isolates within these species, one could reasonably argue that it may have benefited from an efficient horizontal transfer context, making it highly competent to spread.

Occurrence and spread of antibiotic resistances in Enterococcus faecium

Enterococci are the second to third most important bacterial genus in hospital infections. Especially Enterococcus (E.) faecium possesses a broad spectrum of natural and acquired antibiotic resistances which are presented in detail in this paper. From medical point of view, the transferable resistances to glycopeptides (e.g., vancomycin, VAN, or teicoplanin, TPL) and streptogramins (e.g., quinupristin/dalfopristin, Q/D) in enterococci are of special interest. The VanA type of enterococcal glycopeptide resistance is the most important one (VAN-r, TPL-r); its main reservoir is E. faecium. Glycopeptide-resistant E. faecium (GREF) can be found in hospitals and outside of them, namely in European commercial animal husbandry in which the glycopeptide avoparcin (AVO) was used as growth promoter in the past. There are identical types of the vanA gene clusters in enterococci from different ecological origins (faecal samples of animals, animal feed, patients in hospitals, persons in the community, waste water samples). Obviously, across the food chain (by GREF-contaminated meat products), these multiple-resistant bacteria or their vanA gene clusters can reach humans. In hospital infections, widespread epidemic-virulent E. faecium isolates of the same clone with or without glycopeptide resistance can occur; these strains often harbour different plasmids and the esp gene. This indicates that hospital-adapted epidemic-virulent E. faecium strains have picked up the vanA gene cluster after they were already widely spread. The streptogramin virginiamycin was also used as feed additive in commercial animal husbandry in Europe for more than 20 years, and it created reservoirs for streptogramin-resistant E. faecium (SREF). In 1998/1999, SREF could be isolated in Germany from waste water of sewage treatment plants, from faecal samples and meat products of animals that were fed virginiamycin (cross resistance to Q/D), from stools of humans in the community, and from clinical samples. These isolations of SREF occurred in a time before the streptogramin combination Q/D was introduced for therapeutic purposes in German hospitals in May 2000, while other streptogramins were not used in German clinics. This seems to indicate that the origin of these SREF or their streptogramin resistance gene(s) originated from other sources outside the hospitals, probably from commercial animal husbandry. In order to prevent the dissemination of multiple antibiotic-resistant enterococci or their transferable resistance genes, a prudent use of antibiotics is necessary in human and veterinary medicine, and in animal husbandry.

Tetracycline antibiotics: mode of action, applications, molecular biology, and epidemiology of bacterial resistance

Tetracyclines were discovered in the 1940s and exhibited activity against a wide range of microorganisms including gram-positive and gram-negative bacteria, chlamydiae, mycoplasmas, rickettsiae, and protozoan parasites. They are inexpensive antibiotics, which have been used extensively in the prophlylaxis and therapy of human and animal infections and also at subtherapeutic levels in animal feed as growth promoters. The first tetracycline-resistant bacterium, Shigella dysenteriae, was isolated in 1953. Tetracycline resistance now occurs in an increasing number of pathogenic, opportunistic, and commensal bacteria. The presence of tetracycline-resistant pathogens limits the use of these agents in treatment of disease. Tetracycline resistance is often due to the acquisition of new genes, which code for energy-dependent efflux of tetracyclines or for a protein that protects bacterial ribosomes from the action of tetracyclines. Many of these genes are associated with mobile plasmids or transposons and can be distinguished from each other using molecular methods including DNA-DNA hybridization with oligonucleotide probes and DNA sequencing. A limited number of bacteria acquire resistance by mutations, which alter the permeability of the outer membrane porins and/or lipopolysaccharides in the outer membrane, change the regulation of innate efflux systems, or alter the 16S rRNA. New tetracycline derivatives are being examined, although their role in treatment is not clear. Changing the use of tetracyclines in human and animal health as well as in food production is needed if we are to continue to use this class of broad-spectrum antimicrobials through the present century.

Tetracycline antibiotics: mode of action, applications, molecular biology, and epidemiology of bacterial resistance

Tetracyclines were discovered in the 1940s and exhibited activity against a wide range of microorganisms including gram-positive and gram-negative bacteria, chlamydiae, mycoplasmas, rickettsiae, and protozoan parasites. They are inexpensive antibiotics, which have been used extensively in the prophlylaxis and therapy of human and animal infections and also at subtherapeutic levels in animal feed as growth promoters. The first tetracycline-resistant bacterium, Shigella dysenteriae, was isolated in 1953. Tetracycline resistance now occurs in an increasing number of pathogenic, opportunistic, and commensal bacteria. The presence of tetracycline-resistant pathogens limits the use of these agents in treatment of disease. Tetracycline resistance is often due to the acquisition of new genes, which code for energy-dependent efflux of tetracyclines or for a protein that protects bacterial ribosomes from the action of tetracyclines. Many of these genes are associated with mobile plasmids or transposons and can be distinguished from each other using molecular methods including DNA-DNA hybridization with oligonucleotide probes and DNA sequencing. A limited number of bacteria acquire resistance by mutations, which alter the permeability of the outer membrane porins and/or lipopolysaccharides in the outer membrane, change the regulation of innate efflux systems, or alter the 16S rRNA. New tetracycline derivatives are being examined, although their role in treatment is not clear. Changing the use of tetracyclines in human and animal health as well as in food production is needed if we are to continue to use this class of broad-spectrum antimicrobials through the present century.

Erythromycin-resistant Neisseria gonorrhoeae and oral commensal Neisseria spp. carry known rRNA methylase genes

Two Neisseria gonorrhoeae isolates from Seattle and two isolates from Uruguay were resistant to erythromycin (MIC, 4 to 16 microg/ml) and had reduced susceptibility to azithromycin (MIC, 1 to 4 microg/ml) due to the presence of the self-mobile rRNA methylase gene(s) ermF or ermB and ermF. The two Seattle isolates and one isolate from Uruguay were multiresistant, carrying either the 25.2-MDa tetM-containing plasmid (Seattle) or a beta-lactamase plasmid (Uruguay). Sixteen commensal Neisseria isolates (10 Neisseria perflava-N. sicca, 2 N. flava, and 4 N. mucosa) for which erythromycin MICs were 4 to 16 microg/ml were shown to carry one or more known rRNA methylase genes, including ermB, ermC, and/or ermF. Many of these isolates also were multiresistant and carried the tetM gene. This is the first time that a complete transposon or a complete conjugative transposon carrying an antibiotic resistance gene has been described for the genus Neisseria.

Antimicrobial resistance of Neisseria gonorrhoeae in Liberia

The prevalence and molecular characteristics of penicillinase-producing Neisseria gonorrhoeae (PPNG) and tetracycline-resistant N. gonorrhoeae (TRNG) were determined in 10 clinics in Monrovia, Liberia, to assess the likely effectiveness of the current standard treatment with penicillin or tetracycline. One hundred gonococcal strains were isolated from 146 urethral swabs and 261 cervical swabs and screened for susceptibility to ceftriaxone, penicillin, spectinomycin and tetracycline by the disk diffusion method; 83% were resistant to penicillin and 63% to tetracycline. Twenty-one strains from 18 men and 3 women with uncomplicated gonorrhoea were subjected to more detailed characterization. These 21 strains belonged to 5 auxotype/serovar classes; 86% were PPNG/TRNG. Three PPNG harboured the 4.4 MDa penicillinase plasmid and 16 the 3.2 MDa plasmid. All TRNG harboured the 25.2 MDa plasmid and their MICs for tetracycline were > 32 mg/L. They gave a PCR product which, according to its restriction pattern, corresponded to the American type tetM gene. By the agar dilution method, all strains exhibited intermediate resistance to sulphamethoxazole-trimethoprim (19:1) (co-trimoxazole) with MICs of 8-32 mg/L. All strains were susceptible to spectinomycin and ciprofloxacin. The MICs for gentamicin were 4-8 mg/L. The use of effective and affordable antimicrobial chemotherapy with either 500 mg ciprofloxacin or a single dose of gentamicin is discussed, with consideration of molecular biological, pharmacological and public health aspects.

High-level chloramphenicol resistance in Neisseria meningitidis

BACKGROUND

Neisseria meningitidis is nearly always susceptible to the penicillins, the cephalosporins, and chloramphenicol. Between 1987 and 1996, however, chloramphenicol-resistant strains were isolated from 11 patients in Vietnam and 1 in France.

METHODS

The minimal inhibitory concentration of chloramphenicol was determined for the 12 isolates. The isolates were analyzed by monoclonal-antibody-based serotyping and subtyping, pulsed-field gel electrophoresis, and multilocus enzyme electrophoresis. Bacterial DNA was analyzed by hybridization, the polymerase chain reaction, and sequencing to identify the resistance gene and determine the origin of the resistance.

RESULTS

The isolates were resistant to chloramphenicol (minimal inhibitory concentration, > or =64 mg per liter) and produced an active chloramphenicol acetyltransferase. All 12 strains belonged to serogroup B but had a high degree of diversity, and 10 could not be typed with the use of monoclonal antibodies. The nucleotide sequence of the resistance gene and the flanking regions was identical to that of an internal portion of transposon Tn4451 that carries the catP gene in Clostridium perfringens. Moreover, this gene was located in the same genomic site in the chloramphenicol-resistant isolates.

CONCLUSIONS

The high-level chloramphenicol resistance that we describe in N. meningitidis isolates is of great concern, since in developing countries, chloramphenicol given intramuscularly is the standard therapy for meningococcal meningitis. The resistance to chloramphenicol is due to the presence of the catP gene on a truncated transposon that has lost mobility because of internal deletions, and the transformation of genetic material between strains of N. meningitidis probably played an important part in the dissemination of the gene.

Detection of Tn917-like sequences within a Tn916-like conjugative transposon (Tn3872) in erythromycin-resistant isolates of Streptococcus pneumoniae

A series of macrolide-lincosamide-streptogramin B (MLS)-resistant pneumococcal isolates of a variety of serotypes was examined and was found to contain Tn917-like elements by DNA-DNA hybridization. Like Tn1545, Tn917 also encodes an ermAM gene but does not mediate resistance to other antimicrobial agents. Furthermore, nucleotide sequence analyses of the DNAs flanking three of the Tn917-like elements revealed that they were inserted into orf9 of a Tn916-like element in a composite transposon-like structure (Tn3872). Other MLS-resistant strains appeared to contain Tn1545-like elements that had suffered a deletion of sequences including the aphA-3 sequences responsible for kanamycin resistance. Thus, the MLS resistance phenotype in pneumococci appears to be mediated by the ermAM present on a much wider variety of genetic elements than was previously appreciated.

Moraxella catarrhalis: clinical significance, antimicrobial susceptibility and BRO beta-lactamases

Moraxella catarrhalis is an important pathogen of humans. It is a common cause of respiratory infections, particularly otitis media in children and lower respiratory tract infections in the elderly. Colonisation of the upper respiratory tract appears to be associated with infection in many cases, although this association is not well understood. Nosocomial transmission is being increasingly documented and the emergence of this organism as a cause of bacteremia is of concern. The widespread production of a beta-lactamase enzyme renders Moraxella catarrhalis resistant to the penicillins. Cephalosporins and beta-lactamase inhibitor combinations are effective for treatment of beta-lactamase producers, and the organism remains nearly universally susceptible to the macrolides, fluoroquinolones, tetracyclines and the combination of trimethoprim and sulfamethoxazole. Two major beta-lactamase forms, BRO-1 and BRO-2, have been described on the basis of their isoelectric focusing patterns. The BRO-1 enzyme is found in the majority of beta-lactamase-producing isolates and confers a higher level of resistance to strains than BRO-2. The BRO enzymes are membrane associated and their production appears to be mediated by chromosomal determinants which are transmissible by an unknown mechanism. The origin of these novel proteins is unknown.

Diversity among multidrug-resistant enterococci

Enterococci are associated with both community- and hospital-acquired infections. Even though they do not cause severe systemic inflammatory responses, such as septic shock, enterococci present a therapeutic challenge because of their resistance to a vast array of antimicrobial drugs, including cell-wall active agents, all commercially available aminoglycosides, penicillin and ampicillin, and vancomycin. The combination of the latter two occurs disproportionately in strains resistant to many other antimicrobial drugs. The propensity of enterococci to acquire resistance may relate to their ability to participate in various forms of conjugation, which can result in the spread of genes as part of conjugative transposons, pheromone-responsive plasmids, or broad host-range plasmids. Enterococcal hardiness likely adds to resistance by facilitating survival in the environment (and thus enhancing potential spread from person to person) of a multidrug-resistant clone. The combination of these attributes within the genus Enterococcus suggests that these bacteria and their resistance to antimicrobial drugs will continue to pose a challenge.

Pseudo-transposition of a Tn5 derivative in Neisseria gonorrhoeae

We constructed a Tn5 derivative for potential use in transposon mutagenesis of Neisseria gonorrhoeae. It was incorporated into the chromosome apparently at random following transformation, but the insertion events were dependent on a functional RecA and independent of a functional transposase. Furthermore, in most cases there was an incomplete transposon inserted with little or no IS50 insertion sequence. These observations suggest that Tn5 transposition may not be possible in N. gonorrhoeae and that this organism may have an unexplored illegitimate recombination system.

Molecular biology of mycoplasmas

Although mycoplasmas lack cell walls, they are in many respects similar to the gram-positive bacteria with which they share a common ancestor. The molecular biology of mycoplasmas is intriguing because the chromosome is uniquely small (< 600 kb in some species) and extremely A-T rich (as high as 75 mol% in some species). Perhaps to accommodate DNA with a lower G + C content, most mycoplasmas do not have the "universal" genetic code. In these species, TGA is not a stop codon; instead it encodes tryptophan at a frequency 10 times greater than TGG, the usual codon for this amino acid. Because of the presence of TGA codons, the translation of mycoplasmal proteins terminates prematurely when cloned genes are expressed in other eubacteria, such as Escherichia coli. Many mycoplasmas possess strikingly dynamic chromosomes in which high-frequency changes result from errors in DNA repair or replication and from highly active recombination systems. Often, high-frequency changes in the mycoplasmal chromosome are associated with antigenic and phase variation, which regulate the production of factors critical to disease pathogenesis.

Tetracycline resistance determinants: mechanisms of action, regulation of expression, genetic mobility, and distribution

Tetracycline-resistant bacteria were first isolated in 1953 from Shigella dysenteriae, a bacterium which causes bacterial dysentery. Since then tetracycline-resistant bacterial have been found in increasing numbers of species and genera. This has resulted in reduced effectiveness of tetracycline therapy over time. Tetracycline resistance is normally due to the acquisition of new genes often associated with either a mobile plasmid or a transposon. These tetracycline resistance determinants are distinguishable both genetically and biochemically. Resistance is primarily due to either energy-dependent efflux of tetracycline or protection of the ribosomes from the action of tetracycline. Gram-negative tetracycline efflux proteins are linked to repressor proteins which in the absence of tetracycline block transcription of the repressor and structural efflux genes. In contrast, expression of the Gram-positive tetracycline efflux genes and some of the ribosomal protection genes appears to be regulated by attenuation of mRNA transcription. Specific tetracycline resistance genes have been identified in 32 Gram-negative and 22 Gram-positive genera. Tetracycline-resistant bacteria are found in pathogens, opportunistic and normal flora species. Tetracycline-resistant bacteria can be isolated from man, animals, food, and the environment. The nonpathogens in each of these ecosystems may play an important role as reservoirs for the antibiotic resistance genes. It is clear that if we are to reverse the trend toward increasingly antibiotic-resistant pathogenic bacteria we will need to change how antibiotics are used in both human and animal health and food production.

Distribution of tet(H) among Pasteurella isolates from the United States and Canada

Tetracycline-resistant isolates of Pasteurella multocida and Pasteurella haemolytica obtained from various locations in the United States and Canada were studied to determine the distribution of the tet(H) gene. Of the 31 isolates examined, 25 were found to contain the tet(H) gene. Chromosomal or plasmid DNA obtained from those that did not contain the tet(H) gene did not hybridize with probes specific for classes A through G, though chromosomal DNA from one isolate lacking tet(H) hybridized with a probe specific for class M. The tet(H) gene was found on plasmid as well as on chromosomal DNA, suggesting that it is carried on a transposable element.

Conjugative transposons: an unusual and diverse set of integrated gene transfer elements

Conjugative transposons are integrated DNA elements that excise themselves to form a covalently closed circular intermediate. This circular intermediate can either reintegrate in the same cell (intracellular transposition) or transfer by conjugation to a recipient and integrate into the recipient's genome (intercellular transposition). Conjugative transposons were first found in gram-positive cocci but are now known to be present in a variety of gram-positive and gram-negative bacteria also. Conjugative transposons have a surprisingly broad host range, and they probably contribute as much as plasmids to the spread of antibiotic resistance genes in some genera of disease-causing bacteria. Resistance genes need not be carried on the conjugative transposon to be transferred. Many conjugative transposons can mobilize coresident plasmids, and the Bacteroides conjugative transposons can even excise and mobilize unlinked integrated elements. The Bacteroides conjugative transposons are also unusual in that their transfer activities are regulated by tetracycline via a complex regulatory network.

Detection of two groups of 25.2 MDa Tet M plasmids by polymerase chain reaction of the downstream region

Forty-four Neisseria gonorrhoeae, 12 N. meningitidis, four Kingella denitrificans and one Eikenella corrodens carrying 25.2 MDa Tet M plasmids were analysed using polymerase chain reaction (PCR) to the downstream region of the incomplete Tet M transposon. From each isolate, one of two different PCR fragments of approximately 700 or 1600 bp were obtained. The two different sized PCR fragments had > or = 90% DNA sequence identity with Ureaplasma urealyticum Tet M downstream sequences. The difference between the large PCR fragment and the smaller PCR fragment was a deletion of over 800 bp in the smaller fragment. Both PCR fragments were found in plasmids isolated from N. gonorrhoeae and K. denitrificans. The smaller PCR fragment was found in N. meningitidis plasmids and the larger PCR fragment was found in the E. corrodens plasmid.

Characterization of tetracycline resistance in Actinobacillus actinomycetemcomitans

Comparison of susceptibility data for Actinobacillus actinomycetemcomitans has been difficult because of the lack of standard susceptibility testing conditions. In this study, minimum inhibitory concentration to tetracycline was evaluated by comparing different media, air conditions and incubation times. Ten of 22 (45%) A. actinomycetemcomitans isolated from periodontally diseased sites grew on media supplemented with 4 micrograms per ml of tetracycline, but minimum inhibitory concentrations ranged from 0.125 to 8 micrograms/ml depending on the media and condition used. The best results were obtained with brain heart infusion agar (Difco Laboratories, Detroit MI) incubated in 5% CO2 for 48 h. Eighteen (82%) of the A. actinomycetemcomitans isolates hybridized with the Tet B determinant. The Tet B determinant was transferable between A. actinomycetemcomitans isolates as well as a Haemophilus influenzae recipient and appears to be associated with conjugative plasmids.

Chancroid and Haemophilus ducreyi: an update

Haemophilus ducreyi is a fastidious gram-negative bacillus that causes the sexually transmitted infection chancroid. Chancroid is a major genital ulcerative disease in Africa, Southeast Asia, the Caribbean, and Latin America and is of increasing concern in the United States. Genital ulcerative disease and chancroid in particular have been associated with facilitating the transmission of human immunodeficiency virus. The diagnosis of chancroid based on the clinical appearance of the genital lesion or on the isolation of H. ducreyi on selective medium is relatively insensitive. However, recent advances in nonculture diagnostic tests have enhanced our ability to diagnose chancroid. There has been renewed interest in understanding the pathogenesis of H. ducreyi. In vitro and in vivo models have been developed to help identify important virulence determinants. Through the use of biochemical and molecular techniques, macromolecular components that may be important in virulence have been identified.

Presence of the Listeria tetracycline resistance gene tet(S) in Enterococcus faecalis

Two hundred thirty-eight tetracycline- and minocycline-resistant clinical isolates of Enterococcus and Streptococcus spp. were investigated by dot blot hybridization for the presence of nucleotide sequences related to tet(S) (first detected in Listeria monocytogenes BM4210), tet(K), tet(L), tet(M), tet(O), tet(P), and tet(Q) genes. The tet(S) determinant was found in 22 strains of Enterococcus faecalis, associated with tet(M) in 9 of these isolates and further associated with tet(L) in 3 of these strains. tet(M) was detected in all strains of Streptococcus spp. and in all but 10 isolates of Enterococcus spp.; tet(L) was found in 93 enterococci and tet(O) was found in single isolates of E. faecalis and Streptococcus milleri. No hybridization with the tet(K), tet(P), and tet(Q) probes was observed. Transfer of tet(S) by conjugation to E. faecalis or to E. faecalis and L. monocytogenes was obtained from 8 of the 10 E. faecalis strains harboring only this tet gene. Hybridization experiments with DNAs of four donors and of the corresponding transconjugants suggested that tet(S) was located in the chromosome. These results indicate that the genetic support of tet(S) in E. faecalis is different from that in L. monocytogenes, where it is carried by self-transferable plasmids, and confirm the notion of exchange of genetic information between Enterococcus and Listeria spp. in nature.

Decreased susceptibility to penicillin G and Tet M plasmids in genital and anorectal isolates of Neisseria meningitidis

Genital and anorectal isolates of Neisseria meningitidis were characterized, and their antimicrobial susceptibilities were determined. Twelve of 43 isolates demonstrated moderate susceptibility to penicillin G (MIC range, 0.125 to 0.5 microgram/ml). Two isolates were resistant to tetracycline (MIC, > or = 8 micrograms/ml) and contained plasmids of 25.2 MDa.

Deletions of Tn916-like transposons are implicated in tetM-mediated resistance in pathogenic Neisseria

Using the tetM-containing conjugative transposon Tn916 as a mutagenesis tool, we identified two distinct classes of transposon insertions in the meningococcal chromosome. Class I insertions have an intact copy of Tn916 that appears to have transposed by a novel recombinational mechanism, similar to the transposition of conjugative transposons in Gram-positive bacteria. Class II insertions were characterized by deletions of Tn916 but preservation of the tetM determinant. In addition, we identified Class II Tn916-like insertions in the naturally occurring 25.2 MDa tetM-containing plasmids of both Neisseria meningitidis and Neisseria gonorrhoeae. The turncated Tn916-like insertions appeared to be present in the same site in these two plasmids; however, the deletions of the transposon were different. Plasmid sequence adjacent to the truncated transposon in the 25.2 MDa plasmids was found in a tetracycline-sensitive N. gonorrhoeae 24.5 MDa conjugative plasmid. These data suggest that the 25.2 MDa plasmids are the result of one or a series of Class II Tn916-like insertions into 24.5 MDa conjugative plasmids. Class II insertions of Tn916-like transposons are implicated in the dissemination of tetM resistance in pathogenic Neisseria.

Bacterial resistance to tetracycline: mechanisms, transfer, and clinical significance

Tetracycline has been a widely used antibiotic because of its low toxicity and broad spectrum of activity. However, its clinical usefulness has been declining because of the appearance of an increasing number of tetracycline-resistant isolates of clinically important bacteria. Two types of resistance mechanisms predominate: tetracycline efflux and ribosomal protection. A third mechanism of resistance, tetracycline modification, has been identified, but its clinical relevance is still unclear. For some tetracycline resistance genes, expression is regulated. In efflux genes found in gram-negative enteric bacteria, regulation is via a repressor that interacts with tetracycline. Gram-positive efflux genes appear to be regulated by an attenuation mechanism. Recently it was reported that at least one of the ribosome protection genes is regulated by attenuation. Tetracycline resistance genes are often found on transmissible elements. Efflux resistance genes are generally found on plasmids, whereas genes involved in ribosome protection have been found on both plasmids and self-transmissible chromosomal elements (conjugative transposons). One class of conjugative transposon, originally found in streptococci, can transfer itself from streptococci to a variety of recipients, including other gram-positive bacteria, gram-negative bacteria, and mycoplasmas. Another class of conjugative transposons has been found in the Bacteroides group. An unusual feature of the Bacteroides elements is that their transfer is enhanced by preexposure to tetracycline. Thus, tetracycline has the double effect of selecting for recipients that acquire a resistance gene and stimulating transfer of the gene.

Gardnerella vaginalis: characteristics, clinical considerations, and controversies

The clinical significance, Gram stain reaction, and genus affiliation of Gardnerella vaginalis have been controversial since Gardner and Dukes described the organism as the cause of "nonspecific vaginitis," a common disease of women which is now called bacterial vaginosis. The organism was named G. vaginalis when taxonomic studies showed that it was unrelated to bacteria in various genera including Haemophilus and Corynebacterium. Electron microscopy and chemical analyses have elucidated the organism's gram-variable reaction. Controversy over the etiology of bacterial vaginosis was largely resolved by (i) studies using improved media and methods for the isolation and identification of bacteria in vaginal fluids and (ii) standardization of criteria for clinical and laboratory diagnosis. Besides G. vaginalis, Mobiluncus spp., Mycoplasma hominis, and certain obligate anaerobes are now acknowledged as participants in bacterial vaginosis. The finding that G. vaginalis, Mobiluncus spp., and M. hominis inhabit the rectum indicates a potential source of autoinfection in addition to sexual transmission. Extravaginal infections with G. vaginalis are increasingly recognized, especially when the toxic anticoagulant polyanetholesulfonate is omitted from blood cultures and when urine cultures are incubated anaerobically for 48 h. The finding that mares harbor G. vaginalis suggests that an equine model can be developed for studies of Gardnerella pathogenesis.

A Bacteroides tetracycline resistance gene represents a new class of ribosome protection tetracycline resistance

The ribosome protection type of tetracycline resistance (Tcr) has been found in a variety of bacterial species, but the only two classes described previously, Tet(M) and Tet(O), shared a high degree of amino acid sequence identity (greater than 75%). Thus, it appeared that this type of resistance emerged recently in evolution and spread among different species of bacteria by horizontal transmission. We obtained the DNA sequence of a Tcr gene from Bacteroides, a genus of gram-negative, obligately anaerobic bacteria that is phylogenetically distant from the diverse species in which tet(M) and tet(O) have been found. The Bacteroides Tcr gene defines a new class of ribosome protection resistance genes, Tet(Q), and has a deduced amino acid sequence that was only 40% identical to Tet(M) or Tet(O). Like tet(M) and tet(O), tet(Q) appears to have spread by horizontal transmission, but only within the Bacteroides group.

Development of a polymerase chain reaction-based diagnosis of Trichomonas vaginalis

We developed a polymerase chain reaction (PCR)-based test for detecting the protozoan parasite Trichomonas vaginalis. Genomic libraries were constructed from two independent clinical isolates of T. vaginalis. From these libraries, 12 genomic clones were purified, sequenced, and then screened for uniqueness by computer-assisted sequence comparisons. PCR reactions were performed to evaluate eight PCR-primer pairs, including a primer pair that targeted the T. vaginalis ferredoxin gene. All eight primer pairs yielded PCR products of the expected sizes. However, six of the primer pairs amplified their respective target sequences in limited numbers of clinical T. vaginalis isolates, suggesting the presence of significant genomic variability among isolates. An exception was a primer pair, termed TVA5-TVA6, that amplified a 102-bp genomic sequence, termed A6p, in all of 24 clinical isolates. The A6p sequence was not detected by PCR in human DNA or in a wide variety of flagellates, ciliates, or bacteria tested. The A6p sequence appears highly selective for a broad range of T. vaginalis isolates and holds promise for PCR-based diagnosis of the parasite.

Genetic basis of tetracycline resistance in clinical isolates of Listeria monocytogenes

The genetic basis of tetracycline resistance was studied in 25 clinical isolates of Listeria monocytogenes. Resistance to tetracycline was associated with resistance to minocycline and due to the presence of the tet(M) gene in 24 strains. Association of tet(M) with int-Tn, the gene encoding the protein required for the movements of Tn1545-like conjugative transposons, was found in all strains. Cotransfer of tet(M) and int-Tn among L. monocytogenes cells and from L. monocytogenes to Enterococcus faecalis was detected in 7 of the 12 strains studied at frequencies similar to those obtained with the prototype element Tn1545. tet(L), the second most prevalent tetracycline resistance gene in enterococci and streptococci, was detected in the remaining strain, where it was borne by a 5-kb plasmid. These observations indicate that two types of movable genetic elements, transposons and plasmids, in enterococci and streptococci are responsible for emergence of drug resistance in L. monocytogenes.

Tetracycline resistance determinants in streptococcal species isolated from the bovine mammary gland

Seventy-one streptococci isolated from dairy cows with clinical mastitis were tested for tetracycline resistance. Twenty-one (30%) isolates were tetracycline resistant (Tcr), and eight hybridized with the Tet O, one hybridized with the Tet L, and one hybridized with both the Tet L and Tet K determinants. The remaining Tcr isolates did not hybridize with any of the 5 Gram-positive Tet determinants tested. The Tet O determinants were plasmid-mediated, and four selected strains transferred the Tet O determinant at frequencies of 10(-6) to 10(-8). Strains which did not hybridize with known probes were tested for resistance to minocycline. All of the Streptococcus dysgalactiae had low minimum inhibitory concentrations (MICs) for minocycline, while the S. agalactiae and the one S. uberis showed high MICs to minocycline. This suggests that at least two different uncharacterized Tet determinants exist in these isolates, one conferring high resistance to both tetracycline and minocycline and one conferring only tetracycline resistance.

Conjugal transfer of recombinant transposon Tn916 from Escherichia coli to Bacillus stearothermophilus

A gene for thermostable amylase has been inserted at the BstXI site of Tn916. Mating experiments demonstrated that unlike Tn916, the recombinant transposon, designated Tn916A, could transfer from Escherichia coli to Bacillus stearothermophilus BR219 in broth matings, resulting in chromosomal integration of the transposon and expression of the amylase at significant levels.

Transferable Tet M in Fusobacterium nucleatum

Two tetracycline-resistant Fusobacterium nucleatum strains were able to transfer, by conjugation, tetracycline resistance and the Tet M determinant to recipient F. nucleatum, Peptostreptococcus anaerobius, and Enterococcus faecalis strains at a frequency of 10(-1) to 10(-8).