Diagnostic PCR analysis of the occurrence of methicillin and tetracycline resistance genes among Staphylococcus aureus isolates from phase 3 clinical trials of tigecycline for complicated skin and skin structure infections

Staphylococcus aureus in the Processing Environment of Cured Meat Products

The presence of Staphylococcus aureus in six dry-cured meat-processing facilities was investigated. S. aureus was detected in 3.8% of surfaces from five facilities. The occurrence was clearly higher during processing (4.8%) than after cleaning and disinfection (1.4%). Thirty-eight isolates were typified by PFGE and MLST. Eleven sequence types (STs) were defined by MLST. ST30 (32%) and ST12 (24%) were the most abundant. Enterotoxin genes were detected in 53% of isolates. The enterotoxin A gene (sea) was present in all ST30 isolates, seb in one ST1 isolate, and sec in two ST45 isolates. Sixteen isolates harbored the enterotoxin gene cluster (egc) with four variations in the sequence. The toxic shock syndrome toxin gene (tst) was detected in 82% of isolates. Regarding antimicrobial resistance, twelve strains were susceptible to all the antibiotics tested (31.6%). However, 15.8% were resistant to three or more antimicrobials and, therefore, multidrug-resistant. Our results showed that in general, efficient cleaning and disinfection procedures were applied. Nonetheless, the presence of S. aureus with virulence determinants and resistance to antimicrobials, particularly multidrug-resistant MRSA ST398 strains, might represent a potential health hazard for consumers.

Prevalence and Characteristics of Staphylococcus aureus Associated with Meat and Meat Products in African Countries: A Review

Antimicrobial resistance has been increasing globally, which negatively affects food safety, veterinary, and human medicine. Ineffective antibiotics may cause treatment failure, which results in prolonged hospitalisation, increased mortality, and consequently, increased health care costs. Staphylococcus aureus causes a diverse range of infections including septicaemia and endocarditis. However, in food, it mainly causes food poisoning by the production of enterotoxins. With the discovery of methicillin-resistant S. aureus strains that have a separate reservoir in livestock animals, which were termed as livestock-associated methicillin-resistant S. aureus (LA-MRSA) in 2005, it became clear that animals may pose another health risk. Though LA-MRSA is mainly transferred by direct contact, food transmission cannot be excluded. While the current strains are not very pathogenic, mitigation is advisable, as they may acquire new virulence genes, becoming more pathogenic, and may transfer their resistance genes. Control of LA-MRSA poses significant problems, and only Norway has an active mitigation strategy. There is limited information about LA-MRSA, MRSA in general, and other S. aureus infections from African countries. In this review, we discuss the prevalence and characteristics of antimicrobial susceptible and resistant S. aureus (with a focus on MRSA) from meat and meat products in African countries and compare it to the situation in the rest of the world.

First Survey on the Presence and Distribution of Oxytetracycline-Resistance Genes in Anaplasma Species

INTRODUCTION

Anaplasma sp. is an obligatory intracellular Gram-negative tick-transmitted bacterial pathogen of humans and animals. Oxytetracycline and chlortetracycline are the drugs of choice for treating domestic animals with acute anaplasmosis. Lack of documented information about oxytetracycline resistance in Anaplasma species in the world was the scope of this study to screen by PCR for the detection of the oxytetracycline-resistance genes in Anaplasma species from infected cattle and sheep in the Northwest and Southwest of Iran.

MATERIALS AND METHODS

Total of 100 cattle and sheep blood samples collected from 2 provinces in the Northwest and 1 province in the Southwest of Iran were tested microscopically by the Giemsa staining examination and confirmed by PCR. Then the presence of two different oxytetracycline-resistance genes (otrA, and otrB) was detected by PCR in positive samples.

RESULTS

The results showed that 60% of Anaplasma-infected samples were identified to have an otrA-resistance gene, and 26.67% had an otrB-resistance gene. The coexistence of two oxytetracycline-resistance determinants was encountered in 13.33% of the isolates. The significant difference in the frequency of otr genes was found among three Anaplasma species (A. marginale, A. centrale and A. ovis), and among three studied regions in Iran (p < 0.05). The identified sequences were submitted to the GenBank and deposited under accession numbers MN880729 and MN895439 for otrB and otrA genes.

CONCLUSION

This study, for the first time, indicated the oxytetracycline-resistance genes in the three most prevalent Anaplasma species in ruminants. This finding helps to select an appropriate treatment strategy for eradication of anaplasmosis.

In vitro Activity and Heteroresistance of Omadacycline Against Clinical Staphylococcus aureus Isolates From China Reveal the Impact of Omadacycline Susceptibility by Branched-Chain Amino Acid Transport System II Carrier Protein, Na/Pi Cotransporter Family Protein, and Fibronectin-Binding Protein

Omadacycline (Omad), a new tetracycline (Tet)-class broad-spectrum aminomethylcycline, has been reported to exhibit excellent potency against Gram-positive bacteria, including Staphylococcus aureus and Enterococci. The aim of this study was to evaluate the in vitro activity and heteroresistance characteristics of Omad in clinical S. aureus isolates from China and investigate Omad resistance mechanisms. A sample of 263 non-duplicate clinical S. aureus isolates [127 methicillin-resistant (MRSA) and 136 methicillin-sensitive (MSSA)] were collected retrospectively. Our data indicated that Omad exhibited excellent in vitro activity against both MRSA and MSSA. Omad heteroresistance frequencies were 3.17% (4/126) in MRSA and 12.78% (17/133) in MSSA. No mutations in Tet target sites, (five 16SrRNA copies and 30S ribosomal protein S10) were present in heteroresistance-derived clones, whereas Tet target site mutations contribute to induced Omad resistance in S. aureus in vitro. RNA sequencing (RNA-Seq) revealed that overexpression of branched-chain amino acid transport system II carrier protein and Na/Pi cotransporter family protein contributes to Omad heteroresistance emergence. Whole-genome sequencing demonstrated that the genetic mutation of fibronectin-binding protein (FnBP) could increase the Omad MIC. In conclusion, Omad heteroresistance risk should be considered in clinical isolates with MICs ≥ 0.5 mg/L and Omad susceptibility in S. aureus may be affected by efflux pump proteins (i.e., a branched-chain amino acid transport system II carrier protein and an Na/Pi cotransporter family protein), and FnBP.

In vitro antimicrobial susceptibility testing methods: agar dilution to 3D tissue-engineered models

In the field of orthopaedic surgery, bacterial invasion of implants and the resulting periprosthetic infections are a common and unresolved problem. Antimicrobial susceptibility testing methods help to define the optimal treatment and identify antimicrobial resistance. This review discusses proven gold-standard techniques and recently developed models for antimicrobial susceptibility testing, while also providing a future outlook. Conventional, gold-standard methods, such as broth microdilution, are still widely applied in clinical settings. Although recently developed methods based on microfluidics and microdroplets have shown advantages over conventional methods in terms of testing speed, safety and the potential to provide a deeper insight into resistance mechanisms, extensive validation is required to translate this research to clinical practice. Recent optical and mechanical methods are complex and expensive and, therefore, not immediately clinically applicable. Novel osteoblast infection and tissue models best resemble infections in vivo. However, the integration of biomaterials into these models remains challenging and they require a long tissue culture, making their rapid clinical implementation unlikely. A method applicable for both clinical and research environments is difficult to realise. With a continuous increase in antimicrobial resistance, there is an urgent need for methods that analyse recurrent infections to identify the optimal treatment approaches. Graphical abstract Timeline of published and partly applied antimicrobial susceptibility testing methods, listed according to their underlying mechanism, complexity and application in research or clinics.

Genotypic and phenotypic β-lactam resistance and presence of PVL gene in Staphylococci from dry bovine udder

Dairy cows affected with subclinical mastitis can be sources of virulent, antimicrobial-resistant Staphylococci to humans because of the excretion of the bacteria through their milk. This study focussed on the phenotypic and genotypic antibiotic resistance patterns of Staphylococci isolated from dairy cows in early dry period. Among 96 isolates of Gram positive cocci from 157 cows, 76 were identified as Coagulase Negative Staphylococci and the remaining 20 were Staphylococcus aureus. Typical amplicons of coagulase gene were obtained for all 20 samples of S. aureus with three major coagulase types being identified as giving 627 bp (40%), 910 bp (35%) and 710 bp (25%) long PCR products. The groEL gene was amplified in PCR of all 76 isolates of Coagulase Negative Staphylococci, and incubation of PCR products with restriction enzyme PvuII yielded three distinct PCR-RFLP fragment patterns bearing resemblance to S. chromogenes and S. hyicus. Highest sensitivity of Coagulase Negative Staphylococci was noted for Azithromycin (92.5%) and the least to Tetracyclines (76.3%), whereas for S. aureus, it was Cefoperazone (95%) and Azithromycin (72.2%) respectively. Phenotypic resistance to Oxacillin (25 isolates), and Cefoxitin (11 isolates) was detected by dilution method with a commercial strip (Ezy MIC^TM). Genotypic resistance to β-Lactam antibiotics was found in 65 (34 with mecA gene and 31 with blaZ gene) isolates. Eighteen isolates possessed both the genes, with the PVL gene for virulence being detected in five of them. Nine isolates which had mecA gene were phenotypically susceptible to oxacillin while phenotypic resistance to oxacillin was observed in seven isolates that did not have either mecA or blaZ gene. This is the first report of persistent Staphylococcal infections possessing PVL gene and high level of genotypic resistance to β-Lactam antibiotics in small- holder dairy cattle from India.

Detection of benzalkonium chloride resistance in community environmental isolates of staphylococci

We isolated a total of 653 strains from 64 community environmental samples in Massachusetts, USA. Among these isolates, 9.65 % (63 strains) were benzalkonium chloride (BC)-resistant staphylococci. All BC-resistant strains were collected from surfaces upon which antibacterial wipes or antibacterial sprays containing 0.02-0.12 % BC had frequently been used in the fitness centres. However, isolates from surfaces upon which antibacterial wipes or antibacterial sprays had not been used were all sensitive to BC. All BC-resistant strains were also resistant to erythromycin, penicillin and ampicillin. In addition, 51 strains showed resistance to cetyltrimethylammonium bromide (CTAB), 15 strains showed resistance to chloramphenicol, 12 strains showed resistance to ciprofloxacin and four strains showed resistance to meticillin. Resistance gene analysis demonstrated that 41 strains contained qacA/B, 30 strains had qacC, 25 strains contained qacG, 16 strains had qacH and eight strains contained qacJ. These data indicate that application of BC is associated with environmental staphylococcal antimicrobial resistance.

Distribution of genes encoding tetracycline resistance and aminoglycoside modifying enzymes in Staphylococcus aureus strains isolated from a burn center

Staphylococcus aureus (S. aureus) is one of the most common organisms associated with infections among burn patients and has shown a frequent and rapid development of antibiotic resistance. The presence of genes encoding aminoglycoside modifying enzymes (AME) and tetracycline resistance were detected by PCR and multiplex-PCR. Among the 151 S. aureus isolates recovered from the burn patients, 96 (63.6%) were detected to have mecA gene. The rate of tetracycline resistance genes associated with mecA was 61% (92/151). Forty nine isolates (32.4%) contained tetM, 26 (17.2%) possessed only tetK and 21 (13.9%) contained both tetM and tetK. The presence of the aac(6')-Ie-aph(2'')-I gene was determined in 18 isolates, aph(3')-IIIa in 8 isolates, both the aac(6')-Ie-aph(2'')-I, aph(3')-IIIa and the ant(4')-Ia genes in 69 isolates, both aac(6')-Ie-aph(2'')-I and ant(4')-Ia in 6 isolates, and both the aph(3')-IIIa and the ant(4')-Ia genes in 8 isolates. Most of the strains which harboured the mecA gene also contained the tet and AME genes.

ermA, ermC , tetM and tetK are essential for erythromycin and tetracycline resistance among methicillin-resistant Staphylococcus aureus strains isolated from a tertiary hospital in Malaysia

The objective of this study was to determine the expression and transferability of tetracycline and erythromycin resistance among 188 MRSA strains from a Malaysian tertiary hospital. The minimum inhibitory concentrations (MICs) for oxacillin, erythromycin, tetracycline and ciprofloxacin ranged from 4 to 512 μg/ml, 0.25 to 256 μg/ml, 0.5 to 256 μg/ml and 0.5 to 512 μg/ml, respectively. Tetracycline-resistant strains showed co-resistance towards ciprofloxacin and erythromycin. There was a significant increase (P<0.05) of high-level tetracycline (≥MIC 256 μg/ml) and erythromycin (≥MIC 128 μg/ml) resistant strains in between the years 2003 and 2008. All erythromycin-resistant strains harboured ermA or ermC gene and all tetracycline-resistant strains harboured tetM or tetK gene. The blaZ was detected in all MRSA strains, whereas ermA, tetM, ermC, tetK and msrA genes were detected in 157 (84%), 92 (49%), 40 (21%), 39 (21%) and 4 (2%) MRSA strains, respectively. The blaZ, tetM, ermC and tetK genes were plasmid-encoded, with ermC gene being easily transmissible. Tn5801-like transposon was present in 78 tetM-positive strains. ermA and tetM genes were the most prevalent erythromycin and tetracycline resistance determinants, respectively, in MRSA strains. The association of resistance genes with mobile genetic elements possibly enhances the spread of resistant traits in MRSA.

Transferable multidrug resistance plasmid carrying cfr associated with tet(L), ant(4')-Ia, and dfrK genes from a clinical methicillin-resistant Staphylococcus aureus ST125 strain

A multidrug resistance (MDR) conjugative plasmid of ca. 50 kb (designated pERGB) was detected in a linezolid and methicillin-resistant Staphylococcus aureus strain with sequence type 125 (ST125-MRSA-IVc). This strain was detected in two patients with chronic obstructive pulmonary disease, previously treated with multiple antimicrobials, including linezolid. pERGB was transferable by conjugation and carried the resistance genes cfr (oxazolidinones, phenicols, lincosamides, pleuromutilins, and streptogramin A), ant(4')-Ia (tobramycin), tet(L) (tetracycline), and dfrK (trimethoprim). A novel genetic structure, linking all of these resistance genes for the first time, was elucidated through sequencing of a 15,259-bp fragment from pERGB. Active surveillance to prevent the dissemination of such highly concerning MDR transferable elements is needed.

Distribution and antimicrobial resistance of clinical and subclinical mastitis pathogens in dairy cows in Rhône-Alpes, France

The goal of this study was to estimate the distribution of pathogens, as well as their antimicrobial resistance pattern, in cows affected by clinical or subclinical mastitis in the Rhône-Alpes region of France. A total of 1770 samples were taken between January 2007 and March 2008, leading to the identification of 1631 bacterial isolates. Streptococcus uberis (22.1%), Escherichia coli (16%), and coagulase-positive staphylococci (15.8%) were identified as the major causative agents of clinical mastitis, whereas coagulase-positive staphylococci (30.2%), coagulase-negative staphylococci (13.7%), and Streptococcus dysgalactiae (9.3%) were predominantly implicated in subclinical mastitis. Yet, in both types of mastitis, about 20% of all cases were due to a large number of different bacterial species that were isolated at a low frequency (<5%), which cannot be considered as minor (e.g., Klebsiella spp.) or noncontagious (e.g., Corynebacterium spp.). The overall proportion of antibiotic resistance was low, except for penicillin G in staphylococci, as well as for macrolides and tetracycline in streptococci. Yet, these resistance proportions were much lower than those reported in human medicine. Besides providing up-to-date information on mastitis in France, this survey also indicates the prudent use of antibiotics by veterinarians. As a result, this study suggests that the risk of transmission of resistant bacteria from milk or milk products to human is very limited, even in case of consumption of raw milk. However, it also confirms the fact that attention must be maintained to avoid any emergence of such resistant bacteria.

Antimicrobial susceptibility and methicillin resistance in Staphylococcus pseudintermedius and Staphylococcus schleiferi subsp. coagulans isolated from dogs with pyoderma in Japan

To understand species distribution, trends of antimicrobial susceptibility and prevalence of methicillin resistance in canine staphylococci in Japan, 190 coagulase-positive staphylococci (CoPS) were isolated from dogs with pyoderma in 2 Japanese veterinary referral hospitals. Using a multiplex polymerase chain reaction (M-PCR) method, two CoPS species were identified: 170 Staphylococcus pseudintermedius (89.5%) and 20 S. schleiferi subsp. coagulans isolates (10.5%). In these isolates, susceptibility to 7 antimicrobial agents was determined. Overall, the levels of susceptibility to cefalexin (CEX), amoxicillin/clavulanic acid (CVA/AMPC), minocycline (MINO), ofloxacin (OFLX), norfloxacin (NFLX), lincomycin (LCM) and clindamycin (CLDM) in S. pseudintermedius isolates were 38.2, 52.4, 34.7, 31.2, 34.1, 1.2 and 11.2%, respectively. In S. schleiferi subsp. coagulans isolates, 55% demonstrated susceptibility to CEX, 80% to CVA/AMPC, 70% to MINO, 45% to OFLX or NFLX and 30% to CLDM. None of S. schleiferi subsp. coagulans isolates was susceptible to LCM. To determine the prevalence of methicillin-resistant strains, we used a PCR method, which enabled detection of the fragment of mecA gene in 66.5% (113 of 170) in S. pseudintermedius and 30.0% (6 of 20) in S. schleiferi subsp. coagulans isolates. The frequencies of susceptibility to CEX, CVA/AMPC, OFLX, NFLX and CLDM were significantly lower in methicillin-resistant CoPS than in methicillin-susceptible CoPS isolates. These data suggest a high level of methicillin resistance in staphylococci isolated from dogs with pyoderma in Japan.

In vitro activity of tigecycline against patient isolates collected during phase 3 clinical trials for hospital acquired pneumonia

The in vitro activity of tigecycline was evaluated against 819 baseline pathogens isolated from 383 patients enrolled in the phase 3 clinical trial investigating the efficacy of tigecycline in hospital acquired pneumonia (HAP). The trials were global, enrolling patients in 27 countries. Tigecycline was active against the most prevalent pathogens in HAP, including gram-positive and gram-negative strains (90% of MICs ≤2 µg/mL for the entire collection). The spectrum of activity of tigecycline included important pathogens such as Staphylococcus aureus (including methicillin-resistant S. aureus), Enterococcus faecalis, Streptococcus pneumoniae, Acinetobacter baumannii/calcoaceticus complex, Escherichia coli, Klebsiella pneumonia, and Enterobacter cloacae. As reported previously, a few genera, such as Pseudomonas aeruginosa and the Proteeae, were generally less susceptible to tigecycline by comparison to other gram-negative pathogens. The excellent in vitro, expanded, broad-spectrum activity of tigecycline in the clinical isolates confirmed the potential utility of tigecycline for pathogens associated with with hospital acquired pneumonia infections.

Characterization of baseline methicillin-resistant Staphylococcus aureus isolates recovered from phase IV clinical trial for linezolid

A total of 565 methicillin-resistant Staphylococcus aureus (MRSA) isolates were collected mostly from Europe and the Americas (2004 to 2007) during a phase IV clinical trial comparing linezolid with vancomycin for the treatment of complicated skin and skin structure infections proven to be due to MRSA. The isolates were tested for their susceptibilities by the broth microdilution method, they were tested for inducible clindamycin resistance by the D-test, and they were screened for heterogeneous resistance to vancomycin (heterogeneously vancomycin-intermediate S. aureus [hVISA]) by the Etest macromethod. The isolates were evaluated for the MRSA genotype by pulsed-field gel electrophoresis, staphylococcal protein A (spa) typing, multilocus sequence typing (MLST), and staphylococcal cassette chromosome mec (SCCmec) typing. All isolates were inhibited by 4 microg/ml of linezolid (MIC(50) and MIC(90), 2 and 4 microg/ml, respectively). The vast majority of isolates (92.4%) were resistant to erythromycin, and high clindamycin resistance rates were observed (28.5% constitutive and 16.3% inducible). Only 1.0% of the isolates were hVISA. Isolates from the United States were predominantly USA300 sequence type 8 (ST8)-SCCmec type IV (78.5%), followed by a lower prevalence of USA100 ST5-SCCmec type II isolates (14.2%). Strains belonging to the ST5 lineage were widely distributed in Portugal, South American countries, and Mexico. MRSA strains belonging to ST8-SCCmec type IV predominated in Russia (80.0%) and also emerged in Venezuela and Colombia. The epidemic MRSA type 15 clone predominated in the United Kingdom (55.6%) and Spain (100%). In addition, a new MLST profile (ST1071) was observed in South Africa. This study demonstrated the presence of major clones in particular regions (ST8 in the United States, ST5 in Latin America and Portugal, ST22 in Spain and the United Kingdom); however, emerging clones were identified, suggesting that the epidemiology of MRSA continues to evolve.

Efflux-mediated drug resistance in bacteria: an update

Drug efflux pumps play a key role in drug resistance and also serve other functions in bacteria. There has been a growing list of multidrug and drug-specific efflux pumps characterized from bacteria of human, animal, plant and environmental origins. These pumps are mostly encoded on the chromosome, although they can also be plasmid-encoded. A previous article in this journal provided a comprehensive review regarding efflux-mediated drug resistance in bacteria. In the past 5 years, significant progress has been achieved in further understanding of drug resistance-related efflux transporters and this review focuses on the latest studies in this field since 2003. This has been demonstrated in multiple aspects that include but are not limited to: further molecular and biochemical characterization of the known drug efflux pumps and identification of novel drug efflux pumps; structural elucidation of the transport mechanisms of drug transporters; regulatory mechanisms of drug efflux pumps; determining the role of the drug efflux pumps in other functions such as stress responses, virulence and cell communication; and development of efflux pump inhibitors. Overall, the multifaceted implications of drug efflux transporters warrant novel strategies to combat multidrug resistance in bacteria.

Characterization and sequence analysis of extended-spectrum-{beta}-lactamase-encoding genes from Escherichia coli, Klebsiella pneumoniae, and Proteus mirabilis isolates collected during tigecycline phase 3 clinical trials

In concert with the development of novel beta-lactams and broad-spectrum cephalosporins, bacterially encoded beta-lactamases have evolved to accommodate the new agents. This study was designed to identify, at the sequence level, the genes responsible for the extended-spectrum-beta-lactamase (ESBL) phenotypes of Escherichia coli, Klebsiella pneumoniae, and Proteus mirabilis isolates collected during the global tigecycline phase 3 clinical trials. PCR assays were developed to identify and clone the bla(TEM), bla(SHV), bla(OXA), and bla(CTX) genes from clinical strains. Isolates were also screened for AmpC genes of the bla(CMY), bla(ACT), bla(FOX), and bla(DHA) families as well as the bla(KPC) genes encoding class A carbapenemases. E. coli, K. pneumoniae, and P. mirabilis isolates with ceftazidime MICs of > or =2 microg/ml were designated possible ESBL-producing pathogens and were then subjected to a confirmatory test for ESBLs by use of Etest. Of 272 unique patient isolates, 239 were confirmed by PCR and sequencing to carry the genes for at least one ESBL, with 44% of the positive isolates harboring the genes for multiple ESBLs. In agreement with current trends for ESBL distribution, bla(CTX-M)-type beta-lactamase genes were found in 83% and 71% of the ESBL-positive E. coli and K. pneumoniae isolates, respectively, whereas bla(SHV) genes were found in 41% and 28% of the ESBL-positive K. pneumoniae and E. coli isolates, respectively. Ninety-seven percent of the E. coli and K. pneumoniae isolates were tigecycline susceptible (MIC(90) = 2 microg/ml), warranting further studies to define the therapeutic utility of tigecycline against strains producing ESBLs in a clinical setting.

In vitro activity of tigecycline and occurrence of tetracycline resistance determinants in isolates from patients enrolled in phase 3 clinical trials for community-acquired pneumonia

The in vitro activity of tigecycline was evaluated against baseline pathogens isolated from patients enrolled in phase 3 clinical trials for community-acquired pneumonia conducted in 29 countries worldwide. Tigecycline was active against the most prevalent pathogens, including Streptococcus pneumoniae (MIC(90) 0.06 mg/L), Staphylococcus aureus (MIC(90) 0.25 mg/L), Haemophilus influenzae (MIC(90) 0.5 mg/L) and Klebsiella pneumoniae (MIC(90) 1 mg/L). Twelve isolates of S. pneumoniae expressing tet(M) and two isolates of K. pneumoniae producing extended-spectrum beta-lactamases isolated during the study were susceptible to tigecycline. The excellent in vitro activity of tigecycline against these clinical isolates confirmed its potential utility against pathogens associated with community-acquired pneumonia.

Evaluation of the in vitro activity of tigecycline against multiresistant Gram-positive cocci containing tetracycline resistance determinants

This study was undertaken to test the in vitro activity of tigecycline against 117 clinically relevant Gram-positive pathogens and to correlate this activity with their resistance gene content. Overall, tigecycline showed a minimal inhibitory concentration (MIC) range of 0.015-0.5mg/L, able to inhibit potently all multiresistant streptococci, enterococci and MR staphylococci. Tigecycline was active against methicillin-resistant Staphylococcus aureus (MRSA) and enterococci, with MICs for 90% of the organisms (MIC(90)) of 0.25 mg/L and 0.12 mg/L, respectively, being more active than linezolid (MIC(90)=2 mg/L) and quinupristin/dalfopristin (MIC(90)=0.5 and 2-4 mg/L, respectively). Molecular characterisation of resistance determinants demonstrated the concomitant presence of different classes of genes: in particular, tet(M), erm(B) and erm(C) in Streptococcus agalactiae; tet(O), variably associated with different erm genes, in Streptococcus pyogenes; vanA, tet(M) and erm(B) in Enterococcus faecalis, and vanA and erm(B) in Enterococcus faecium. All the MRSA strains harboured SCCmec and erm genes and 50% possessed tet(K) with tet(M) genes. Staphylococcus epidermidis strains were only resistant to erythromycin. These results clearly demonstrate that tigecycline has a MIC(90) range of 0.015-0.5 mg/L both against tetracycline-susceptible and -resistant isolates carrying other resistance determinants, suggesting that this drug could play an important role in the treatment of infections caused by these multiresistant Gram-positive pathogens.

Antimicrobial activity of tigecycline against community-acquired methicillin-resistant Staphylococcus aureus isolates recovered from North American medical centers

A total of 1989 community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) were susceptibility tested by broth microdilution. Pulsed-field gel electrophoresis, SCCmec type, and polymerase chain reaction for Panton-Valentine leukocidin (PVL) genes were also performed. The overall tigecycline susceptibility rate was 98.2%. Glycopeptides, quinupristin/dalfopristin, linezolid, and chloramphenicol were also active against this collection (< or =0.7% resistant). The vast majority (70.8%) of the CA-MRSA was SCCmec type IV, from which 88.4% belonged to the USA300-0114 clone and 94.7% were PVL positive. Tigecycline showed in vitro activity comparable with other highly active parenteral agents and represents an option for treating complicated infections caused by CA-MRSA.

Therapeutic applications of tigecycline in the management of complicated skin and skin structure infections

Complicated skin and skin structure infections encompass a diverse range of diseases frequently caused by Gram-positive pathogens, and most commonly by Staphylococcus aureus and Streptococcus pyogenes. Treatment of these infections represents a growing clinical challenge as increases in multi-drug-resistant organisms and cross-resistance to antimicrobial therapy have made empiric therapeutic choices more difficult, particularly for patients with known risk factors or who are immunocompromised. Complicating this issue has been the relative lack of new agents with antimicrobial potency against prevalent resistant species such as meticillin resistant S. aureus (MRSA). Tigecycline, a novel glycylcycline, is a broad-spectrum antibiotic with potent microbiological activity against the wide variety of organisms implicated in the aetiology of complicated skin and skin structure infections. Recent phase III clinical data confirm previous observations on the safety and efficacy of tigecycline for the treatment of complicated skin and skin structure infections. Tigecycline was shown to be non-inferior to combination vancomycin-aztreonam regimens and exhibited high clinical success rates. MIC(90) values for tigecycline were uniformly low for both susceptible and resistant pathogens. Adverse events were similar in incidence for both patient populations, with nausea and vomiting reported more frequently with tigecycline treated patients while rash and elevated liver transaminases were most commonly observed in the vancomycin-aztreonam treatment group. Tigecycline helps to address the urgent need for new antimicrobial agents to combat the emergence of multi-drug-resistant Gram-positive pathogens. Current clinical, microbiological and safety data support the use of tigecycline as a valuable therapeutic option in the treatment of complicated skin and skin structure infections.

Occurrence of tetracycline resistance genes among Escherichia coli isolates from the phase 3 clinical trials for tigecycline

Tigecycline, a member of the glycylcycline class of antibiotics, was designed to maintain the antibacterial spectrum of the tetracyclines while overcoming the classic mechanisms of tetracycline resistance. The current study was designed to monitor the prevalence of the tet(A), tet(B), tet(C), tet(D), tet(E), and tet(M) resistance determinants in Escherichia coli isolates collected during the worldwide tigecycline phase 3 clinical trials. A subset of strains were also screened for the tet(G), tet(K), tet(L), and tet(Y) genes. Of the 1,680 E. coli clinical isolates screened for resistance to classical tetracyclines, 405 (24%) were minocycline resistant (MIC > or = 8 microg/ml) and 248 (15%) were tetracycline resistant (MIC > or = 8 microg/ml) but susceptible to minocycline (MIC < or = 4 microg/ml). A total of 452 tetracycline-resistant, nonduplicate isolates were positive by PCR for at least one of the six tetracycline resistance determinants examined. Over half of the isolates encoding a single determinant were positive for tet(A) (26%) or tet(B) (32%) with tet(C), tet(D), tet(E), and tet(M), collectively, found in 4% of isolates. Approximately 33% of the isolates were positive for more than one resistance determinant, with the tet(B) plus tet(E) combination the most highly represented, found in 11% of isolates. The susceptibilities of the tetracycline-resistant strains to tigecycline (MIC(90), 0.5 microg/ml), regardless of the encoded tet determinant(s), were comparable to the tigecycline susceptibility of tetracycline-susceptible strains (MIC(90), 0.5 microg/ml). The results provide a current (2002 to 2006) picture of the distribution of common tetracycline resistance determinants encoded in a globally sourced collection of clinical E. coli strains.

Validation and reproducibility assessment of tigecycline MIC determinations by Etest

A multicenter study was conducted to validate Etest tigecycline compared to the Clinical Laboratory Standards Institute reference broth microdilution and agar dilution methodologies. A large collection of gram-negative (n = 266) and gram-positive (n = 162) aerobic bacteria, a collection of anaerobes (n = 385), and selected collections of nonpneumococcal streptococci (n = 369), Streptococcus pneumoniae (n = 372), and Haemophilus influenzae (n = 372) were tested. Strains with reduced susceptibility to tigecycline were used with all test methods. The Etest showed excellent inter- and intralaboratory reproducibility for all organism groups tested regardless of the test methodology. The essential agreement values with the reference method (+/-1 dilution) were >99% for the collection of gram-negative and gram-positive aerobes; >98% for the S. pneumoniae, H. influenzae, and anaerobe collections; and 100% for the group of nonpneumococcal streptococci. These results validate the performance accuracy and utility of Etest tigecycline and verify the reproducibility of this convenient predefined gradient methodology for tigecycline susceptibility determination.

RamA, a Transcriptional Regulator, and AcrAB, an RND-Type Efflux Pump, are Associated with Decreased Susceptibility to Tigecycline inEnterobacter cloacae

Tigecycline, a novel broad-spectrum glycylcycline antibiotic, is active against many gram-positive and gram-negative bacterial pathogens including most strains of Enterobacter cloacae. Recently, however, a few clinical strains of E. cloacae with decreased susceptibility to tigecycline were isolated. In this study, two tigecycline-susceptible mutants of E. cloacae, GC7696 and GC7697, were obtained by transposon mutagenesis of a tigecycline-resistant clinical isolate G946. Transposon insertions were mapped to either the acrA or acrB genes. Restoration of the original resistant phenotype occurred when GC7696 and GC7697 were transcomplemented with a plasmid harboring the intact acrAB region amplified from G946. Northern blot analysis of G946 and several other E. cloacae clinical strains that exhibited decreased susceptibility to tigecycline, revealed increased levels of the acrAB transcript. In addition, overexpression of acrAB correlated with increased expression of the ramA gene, whereas the expression of another transcriptional activator, marA, was not changed. These results suggest that decreased susceptibility to tigecycline in E. cloacae is the result of RamA-mediated overexpression of the AcrAB efflux pump.

Emerging Options for Treatment of Invasive, Multidrug-ResistantStaphylococcus aureusInfections

Limited established treatment options exist for the treatment of serious, invasive infections caused by multidrug-resistant Staphylococcus aureus, most notably nosocomially acquired methicillin-resistant S. aureus (MRSA). Although vancomycin represents the gold standard for therapy of such invasive infections, reports of increasing in vitro resistance to vancomycin, combined with reports of clinical failures (with this and other antistaphylococcal agents), underscore the need for alternative therapies. Older agents with favorable in vitro activity available in both oral and intravenous dose forms include trimethoprim-sulfamethoxazole and clindamycin. Limited clinical data exist to support their routine use as initial therapy in the treatment of invasive disease. However, these and other options (e.g., tetracyclines) are being reexplored in the setting of increasing concern over MRSA acquired in the community setting. Newer treatment options for MRSA include linezolid, quinupristin-dalfopristin, daptomycin, and tigecycline. With the exception of linezolid, these newer agents require intravenous administration. Combination therapy may be considered in select invasive diseases refractory to standard monotherapies. These diseases include infections such as endocarditis, meningitis, and prosthetic device infections. Additional alternatives to vancomycin are under clinical investigation. Those in later stages of development include oritavancin, dalbavancin, telavancin, and ceftobiprole.

[Epidemiology, control and treatments of antimicrobial resistances: highlights of the 45th ICAAC, Washington, 2005]

For more than 20 years, hospital and community-acquired antimicrobial resistances regularly increase. In France, methicilline-resistant Staphylococcus aureus (MRSA) and extended-spectrum beta-lactamase-producing Enterobacteriaceae are the most common resistant pathogens. Previous hospitalisation, surgery, long length of stay, enteral feeding and antibiotic exposure are the main risk factors associated with nosocomial MRSA infections. To limit the emergence of resistances, control strategies have been implemented, based on isolation practices, healthcare workers education, strict hand hygiene and a controlled use of antibiotics. However, new antimicrobials acting by a novel mechanism of action are necessary in fighting the most resistant organisms. Therefore, the launch on the American market in June 2005 of tigecycline, a first-in-class glycylcycline indicated in the treatment of complicated skin and skin structure infections and complicated intra-abdominal infections, will provide to physicians a new therapeutic option against hospital or community-acquired resistant pathogens. The data presented to ICAAC indicate its expended broad spectrum of in vitro activity against Gram positives, Gram negatives, anaerobes and against number of difficult-to-treat resistant isolates, such as hospital or community-acquired MRSA, vancomycine resistant Enterococcus but also ESBL-producing organisms.