Staphylococcus efflux msr(A) gene characterized in Streptococcus, Enterococcus, Corynebacterium, and Pseudomonas isolates

Multi-Host Pathogen Staphylococcus aureus-Epidemiology, Drug Resistance and Occurrence in Humans and Animals in Poland

Staphylococcus aureus is a drug resistant pathogen with zoonotic potential commonly isolated from humans and animals. The aim of this study was to compare the occurrence of drug resistance, resistance genes, sequence types (STs), and genotypes of S. aureus isolated from humans, livestock, and wildlife in eastern Poland. A high percentage of isolates resistant to penicillin (63%), erythromycin (39%), clindamycin (37%), tetracycline (31%), and methicillin (MRSA-19%), an intermediate resistant to vancomycin (VISA-13%), and a multidrug resistant (MDR-39%) was obtained. Multilocus sequence typing analysis showed the presence of 35 different STs (with dominance ST 15, ST 45, ST 7, and ST 582 in human, and ST 398 and ST 8139 in porcine and cattle isolates, respectively), including 9 new ones that had never been reported before (ST 8133-8141). Identical genotypic patterns were detected among porcine and cattle isolates as well as from humans and cattle. A high percentage of MDR, MRSA, and VISA in humans and livestock combined with the presence of the same genotypes among S. aureus isolated from human and cattle indicates the circulation of strains common in the region and their zoonotic potential. There is a need to develop new strategies to counteract this phenomenon according to the One Health policy.

Phenotypic and genotypic characterization of linezolid resistance and the effect of antibiotic combinations on methicillin-resistant Staphylococcus aureus clinical isolates

BACKGROUND

Methicillin-Resistant Staphylococcus aureus (MRSA) causes life-threatening infections, with narrow therapeutic options including: vancomycin and linezolid. Accordingly, this study aimed to characterize phenotypically and genotypically, the most relevant means of linezolid resistance among some MRSA clinical isolates.

METHODS

A total of 159 methicillin-resistant clinical isolates were collected, of which 146 were indentified microscopically and biochemically as MRSA. Both biofilm formation and efflux pump activity were assessed for linezolid-resistant MRSA (LR-MRSA) using the microtiter plate and carbonyl cyanide 3-chlorophenylhydrazone (CCCP) methods, respectively. Linezolid resistance was further characterized by polymerase chain reaction (PCR) amplification and sequencing of domain V of 23 S rRNA; rplC; rplD;and rplV genes. Meanwhile, some resistance genes were investigated: cfr; cfr(B); optrA; msrA;mecA; and vanA genes. To combat LR-MRSA, the effect of combining linezolid with each of 6 different antimicrobials was investigated using the checkerboard assay.

RESULTS

Out of the collected MRSA isolates (n = 146), 5.48% (n = 8) were LR-MRSA and 18.49% (n = 27) were vancomycin-resistant (VRSA). It is worth noting that all LR-MRSA isolates were also vancomycin-resistant. All LR-MRSA isolates were biofilm producers (r = 0.915, p = 0.001), while efflux pumps upregulation showed no significant contribution to development of resistance (t = 1.374, p = 0.212). Both mecA and vanA genes were detected in 92.45% (n = 147) and 6.92% (n = 11) of methicillin-resistant isolates, respectively. In LR-MRSA isolates, some 23 S rRNA domain V mutations were observed: A2338T and C2610G (in 5 isolates); T2504C and G2528C (in 2 isolates); and G2576T (in 1 isolate). Amino acids substitutions were detected: in L3 protein (rplC gene) of (3 isolates) and in L4 protein (rplD gene) of (4 isolates). In addition, cfr(B) gene was detected (in 3 isolates). In 5 isolates, synergism was recorded when linezolid was combined with chloramphenicol, erythromycin, or ciprofloxacin. Reversal of linezolid resistance was observed in some LR-MRSA isolates when linezolid was combined with gentamicin or vancomycin.

CONCLUSIONS

LR-MRSA biofilm producers' phenotypes evolved in the clinical settings in Egypt. Various antibiotic combinations with linezolid were evaluated in vitro and showed synergistic effects.

Clinical Resistant Strains of Enterococci and Their Correlation to Reduced Susceptibility to Biocides: Phenotypic and Genotypic Analysis of Macrolides, Lincosamides, and Streptogramins

Enterococci are troublesome nosocomial, opportunistic Gram-positive cocci bacteria showing enhanced resistance to many commonly used antibiotics. This study aims to investigate the prevalence and genetic basis of antibiotic resistance to macrolides, lincosamides, and streptogramins (MLS) in Enterococci, as well as the correlation between MLS resistance and biocide resistance. From 913 clinical isolates collected from King Khalid Hospital, Hail, Saudi Arabia, 131 isolates were identified as Enterococci spp. The susceptibility of the clinical enterococcal isolates to several MLS antibiotics was determined, and the resistance phenotype was detected by the triple disk method. The MLS-involved resistance genes were screened in the resistant isolates. The current results showed high resistance rates to MLS antibiotics, and the constitutive resistance to all MLS (cMLS) was the most prevalent phenotype, observed in 76.8% of resistant isolates. By screening the MLS resistance-encoding genes in the resistant isolates, the erythromycin ribosome methylase (erm) genes that are responsible for methylation of bacterial 23S rRNA were the most detected genes, in particular, ermB. The ereA esterase-encoding gene was the most detected MLS modifying-encoding genes, more than lnuA (adenylation) and mphC (phosphorylation). The minimum inhibitory concentrations (MICs) of commonly used biocides were detected in resistant isolates and correlated with the MICs of MLS antibiotics. The present findings showed a significant correlation between MLS resistance and reduced susceptibility to biocides. In compliance with the high incidence of the efflux-encoding genes, especially mefA and mefE genes in the tolerant isolates with higher MICs to both MLS antibiotics and biocides, the efflux of resistant isolates was quantified, and there was a significant increase in the efflux of resistant isolates with higher MICs as compared to those with lower MICs. This could explain the crucial role of efflux in developing cross-resistance to both MLS antibiotics and biocides.

Effect of Quorum Sensing Molecule Farnesol on Mixed Biofilms of Candida albicans and Staphylococcus aureus

The natural bioactive molecule farnesol (FAR) is widely studied mainly for its antibiofilm and antimicrobial properties. In addition, it increases the effectiveness of some antimicrobial substances, which makes it interesting for the development of combined therapy. In the present work, the effect of FAR either alone or in combination with oxacillin (OXA) on mixed biofilms formed by clinically relevant pathogens, Candida albicans and Staphylococcus aureus, was studied. S. aureus isolates used for biofilm formation originated from blood cultures and central venous catheters (CVC) were characterized in terms of antimicrobial resistance. The minimal biofilm inhibitory concentration (MBIC50) for FAR of 48 h mixed biofilms formed by the C. albicans and methicillin-sensitive S. aureus (MSSA) was determined to be 125 μM, and for the mixed biofilms with methicillin-resistant S. aureus (MRSA) was determined to be 250 μM. Treatment of mixed biofilms with OXA (2 mg/mL) showed ≤4% inhibition; however, the combination of OXA (2 mg/mL) and FAR (300 μM) resulted in 80% inhibition of biofilms. In addition, planktonic cells of S. aureus exhibited an increased susceptibility to OXA, cefoxitin and kanamycin in the presence of FAR (150 and 300 μM). Scanning electron microscopy (SEM) micrographs confirmed patchy biofilm and lack of candidal hyphae in the samples treated with FAR and FAR/OXA in comparison to control and mixed biofilms treated only with OXA. Intriguingly, in a pilot experiment using fluorescence in situ hybridization (FISH), considerable differences in activity (as indicated by ribosome content) of staphylococcal cells were detected. While the activity rate of the staphylococci in mixed biofilms treated with FAR was high, no FISH-positive signal for staphylococcal cells was found in the biofilm treated with FAR/OXA.

Identification and characterization of the causative agents of Focal Ulcerative Dermatitis in commercial laying hens

Focal Ulcerative Dermatitis (FUDS) is an emerging dermatological disease that affects cage-free laying flocks, it is characterized by the development of a lesion on the dorsum of the birds; FUDS is sporadic in nature and can result in a drop in egg production and up to 50% of cumulative mortality. A total of two cage-free flocks (flock 1: no history of FUDS; flock 2: birds affected with FUDS) from a commercial laying hen operation in the mid-west U.S. were sampled in this study. The microbial composition of skin, cloacal, cecal, and ileal samples from each bird was characterized through next generation sequencing (NGS). Results identified Staphylococcus aureus and Staphylococcus agnetis as the potential causative agents of FUDS, being the most predominant in FUDS positive birds. These results were confirmed by plating, with both staphylococci as the only pathogens isolated from lesions of FUDS positive birds. A total of 68 confirmed Staphylococcus isolates from skin and environmental samples were further analyzed by whole genome sequencing (WGS) for the presence of antimicrobial resistance (AMR) genes and virulence factors that could have contributed to the development of FUDS. Forty-four-point one-two percent of the isolates had between one and four acquired AMR genes encoding for macrolides, lincosamides, spectrogramines, and beta-lactams resistance. Six classes of virulence factors associated with adherence, enzyme, immune evasion, secretion system, toxin, and iron uptake were identified. The antimicrobial effect of 4 proprietary Bacillus Direct Fed Microbial (DFM) combinations was evaluated against the Staphylococcus aureus and Staphylococcus agnetis isolates, by agar well-diffusion (AWD) assay and competitive exclusion (CE) on broth culture. Through this antimicrobial screening, a particular two-strain combination of Bacillus pumilus was identified as the most effective inhibitor of both staphylococci. A customized Bacillus pumilus product is being used at different farms with history of FUDS resulting in the successful inhibition of both Staphylococcus aureus and Staphylococcus agnetis, decreasing FUDS mortalities, and improving harvestable eggs.

Prevalence of macrolide-lincosamide-streptogramin resistant lactic acid bacteria isolated from food samples

Lactic acid bacteria (LAB) being a reservoir of antibiotic resistance genes, tend to disseminate antibiotic resistance that possibly pose a threat to human and animal health. Therefore, the study focuses on the prevalence of macrolide-lincosamide-streptogramin- (MLS) resistance among LAB isolated from various food samples. Diverse phenotypic and genotypic MLS resistance were determined among the LAB species (n = 146) isolated from fermented food products (n = 6) and intestine of food-producing animals (n = 4). Double disc, triple disc diffusion and standard minimum inhibitory concentration (MIC) tests were evaluated for phenotypic MLS resistance. Specific primers for MLS resistance genes were used for the evaluation of genotypic MLS resistance and gene expressions using total RNA of each isolate at different antibiotic concentrations. The isolates identified are Levilactobacillus brevis (n = 1), Enterococcus hirae (n = 1), Limosilactobacillus fermentum (n = 2), Pediococcus acidilactici (n = 3), Enterococcus faecalis (n = 1). The MIC tests along with induction studies displayed cMLSb, L phenotype, M phenotype, KH phenotype, I phenotype resistance among MLS antibiotics. Genotypic evaluation tests revealed the presence of ermB, mefA/E, msrA/B and msrC genes. Also, gene expression studies displayed increased level of gene expression to the twofold increased antibiotic concentrations. In the view of global health concern, this study identified that food samples and food-producing animals represent source of antibiotic resistant LAB that can disseminate resistance through food chain. This suggests the implementation of awareness in the use of antibiotics as growth promoters and judicious use of antibiotics in veterinary sectors in order to prevent the spread of antibiotic resistance.

The Preyssler-Type Polyoxotungstate Exhibits Anti-Quorum Sensing, Antibiofilm, and Antiviral Activities

The increase in bacterial resistance to antibiotics has led researchers to find new compounds or find combinations between different compounds with potential antibacterial action and with the ability to prevent the development of antibiotic resistance. Polyoxotungstates (POTs) are inorganic clusters that may fulfill that need, either individually or in combination with antibiotics. Herein, we report the ability of the polyoxotungstates (POTs) with Wells-Dawson P2W18, P2W17, P2W15, and Preyssler P5W30 type structures to differently affect Gram-negative and Gram-positive microorganisms, either susceptible or resistant to antibiotics. The compound P5W30 showed the highest activity against the majority of the tested bacterial strains in comparison with the other tested POTs (P2W15, P2W17 and P2W18) that did not show inhibition zones for the Gram-negative bacteria, A. baumanii I73775, E. coli DSM 1077, E. coli I73194, K. pneumoniae I7092374, and P. aeruginosa C46281). Generally, the results evidenced that Gram-positive bacteria are more susceptible to the POTs tested. The compound P5W30 was the one most active against S. aureus ATCC 6538 and MRSA16, reaching <0.83 mg·mL−1 (100 μM) and 4.96 mg·mL−1 (600 μM), respectively. Moreover, it was verified by NMR spectroscopy that the most promising POT, P5W30, remains intact under all the experimental conditions, after 24 h at 37 °C. This prompted us to further evaluate the anti-quorum sensing activity of P5W30 using the biosensor Chromobacterium violaceum CV026, as well as its antibiofilm activity both individually and in combination with the antibiotic cefoxitin against the methicillin-resistant Staphylococcus aureus 16 (MRSA16). P5W30 showed a synergistic antibacterial effect with the antibiotic cefoxitin and chloramphenicol against MRSA16. Moreover, the antibiofilm activity of P5W30 was more pronounced when used individually, in comparison with the combination with the antibiotic cefoxitin. Finally, the antiviral activity of P5W30 was tested using the coliphage Qβ, showing a dose-dependent response. The maximum inactivation was observed at 750 μM (6.23 mg·mL−1). In sum, P5W30 shows anti-quorum sensing and antibiofilm activities besides being a potent antibacterial agent against S. aureus and to exhibit antiviral activities against enteric viruses.

Assessing the drug resistance profiles of oral probiotic lozenges

Background

Probiotic lozenges have been developed to harvest the benefits of probiotics for oral health, but their long-term consumption may encourage the transfer of resistance genes from probiotics to commensals, and eventually to disease-causing bacteria.

Aim

To screen commercial probiotic lozenges for resistance to antibiotics, characterize the resistance determinants, and examine their transferability in vitro.

Results

Probiotics of all lozenges were resistant to glycopeptide, sulfonamide, and penicillin antibiotics, while some were resistant to aminoglycosides and cephalosporins. High minimum inhibitory concentrations (MICs) were detected for streptomycin (>128 µg/mL) and chloramphenicol (> 512 µg/mL) for all probiotics but only one was resistant to piperacillin (MIC = 32 µg/mL). PCR analysis detected erythromycin (erm(T), ermB or mefA) and fluoroquinolone (parC or gyr(A)) resistance genes in some lozenges although there were no resistant phenotypes. The dfrD, cat-TC, vatE, aadE, vanX, and aph(3")-III or ant(2")-I genes conferring resistance to trimethoprim, chloramphenicol, quinupristin/dalfopristin, vancomycin, and streptomycin, respectively, were detected in resistant probiotics. The rifampicin resistance gene rpoB was also present. We found no conjugal transfer of streptomycin resistance genes in our co-incubation experiments.

Conclusion

Our study represents the first antibiotic resistance profiling of probiotics from oral lozenges, thus highlighting the health risk especially in the prevailing threat of drug resistance globally.

Mechanisms of Resistance to Macrolide Antibiotics among Staphylococcus aureus

Methicillin resistant Staphylococcus aureus strains pose a serious treatment problem because of their multi-drug resistance (MDR). In staphylococcal strains, resistance to macrolides, lincosamides, and streptogramin B (MLS_B) correlates with resistance to methicillin. The rapid transmission of erm genes responsible for MLS_B resistance has strongly limited the clinical application of traditional macrolides such as erythromycin. On the other hand, in the age of increasing insensitivity to antibiotics the idea of implementing a therapy based on older generation drugs brings hope that the spread of antibiotic resistance will be limited. A thorough understanding of the resistance mechanisms contributes to design of antibiotics that avoid bacterial insensitivity. This review highlights the mechanisms of action of macrolides and mechanism of resistance to these antibiotics among Staphylococcus aureus.

Impact of Intrapartum Oral Azithromycin on the Acquired Macrolide Resistome of Infants' Nasopharynx: A Randomized Controlled Trial

In a post hoc analysis of samples from an intrapartum azithromycin randomized clinical trial, we found that children whose mothers had been treated with the drug had higher prevalence of macrolide-resistance genes msr(A) and ermC at 28 days but not at 12 months. The 2 genes were positively associated in the nasopharynx.

CLINICAL TRIALS REGISTRATION

NCT1800942.

Comparative characterisation of human and ovine non-aureus staphylococci isolated in Sardinia (Italy) for antimicrobial susceptibility profiles and resistance genes

We present the comparative characterisation of 195 non-aureus staphylococci (NAS) isolates obtained from sheep (n = 125) and humans (n = 70) in Sardinia, Italy, identified at the species level by gap gene polymerase chain reaction (PCR) followed by restriction fragment length polymorphism analysis with AluI. Isolates were tested phenotypically with a disc diffusion method and genotypically by PCR, for resistance to 11 antimicrobial agents including cationic antiseptic agents. Among the ovine isolates, Staphylococcus epidermidis (n = 57), S. chromogenes (n = 29), S. haemolyticus (n = 17), S. simulans (n = 8) and S. caprae (n = 6) were the most prevalent species, while among human isolates, S. haemolyticus (n = 28) and S. epidermidis (n = 26) were predominant, followed by S. lugdunensis and S. hominis (n = 4). Of the 125 ovine isolates, 79 (63.2%) did not carry any of the resistance genes tested, while the remainder carried resistance genes for at least one antibiotic. The highest resistance rates among ovine isolates were recorded against tetracycline (20.8%), and penicillin (15.2%); none was resistant to methicillin and two exhibited multidrug resistance (MDR); one of which was positive for the antiseptic resistance smr gene. By contrast, most human isolates (59/70, 84.3%) were resistant to ⩾1 antimicrobials, and 41 (58.6%) were MDR. All 52 (74.3%) penicillin-resistant isolates possessed the blaZ gene, and 33 of 70 (47.1%) harboured the mec gene; of these, seven were characterised by the Staphylococcal Chromosomal Cassette (SCCmec) type IV, 6 the type V, 5 of type III and one representative each of type I and type II. The majority (57.1%) was erythromycin-resistant and 17 isolates carried only the efflux msrA gene, 11 the methylase ermC gene and an equal number harboured both of the latter genes. Moreover, 23 (32.8%) were tetracycline-resistant and all but one possessed only the efflux tetK gene. qacA/B and smr genes were detected in 27 (38.6%) and 18 (25.7%) human NAS, respectively. These results underline a marked difference in species distribution and antimicrobial resistance between ovine and human-derived NAS.

Phenylpiperazine 5,5-Dimethylhydantoin Derivatives as First Synthetic Inhibitors of Msr(A) Efflux Pump in Staphylococcus epidermidis

Herein, 15 phenylpiperazine 3-benzyl-5,5-dimethylhydantoin derivatives (1-15) were screened for modulatory activity towards Msr(A) efflux pump present in S. epidermidis bacteria. Synthesis, crystallographic analysis, biological studies in vitro and structure-activity relationship (SAR) analysis were performed. The efflux pump inhibitory (EPI) potency was determined by employing ethidium bromide accumulation assay in both Msr(A) efflux pump overexpressed (K/14/1345) and deficient (ATCC 12228) S. epidermidis strains. The series of compounds was also evaluated for the capacity to reduce the resistance of K/14/1345 strain to erythromycin, a known substrate of Msr(A). The study identified five strong modulators for Msr(A) in S. epidermidis. The 2,4-dichlorobenzyl-hydantoin derivative 9 was found as the most potent EPI, inhibiting the efflux activity in K/14/1345 at a concentration as low as 15.63 µM. Crystallography-supported SAR analysis indicated structural properties that may be responsible for the activity found. This study identified the first synthetic compounds able to inhibit Msr(A) efflux pump transporter in S. epidermidis. Thus, the hydantoin-derived molecules found can be an attractive group in search for antibiotic adjuvants acting via Msr(A) transporter.

New Gene Responsible for Resistance of Clinical Corynebacteria to Macrolide, Lincosamide and Streptogramin B

The subject of the study was phenotypic marking of the antibiotic susceptibility and MLS_B resistance mechanism in Corynebacterium spp. isolated from human skin (18 isolates) and from clinical materials (19 isolates). The strains were tested for the presence of the erm(A), erm(B), erm(C), erm(X), lnu(A), msr(A), msr(B) and mph(C) genes. Clinical isolates showed wide resistance to antibiotics. In 89% clinical isolates and 72% skin microbiota a constitutive type of MLS_B resistance was found. In 12 clinical isolates the erm(C) gene was detected-eight of which had erm(X) as well as erm(C), two harboured erm(X), erm(C) and erm(A) and two demonstrated only erm(C).

Azithromycin resistance levels and mechanisms in Escherichia coli

Despite azithromycin being used in some countries to treat infections caused by Gram-negative pathogens, no resistance breakpoint for Escherichia coli exists. The aim of this study was to analyse the levels and mechanisms of azithromycin resistance in E. coli. The presence of chromosomal (rplD, rplV and 23S rRNA) mutations, 10 macrolide resistance genes (MRGs) and efflux pump overexpression was determined in 343 E. coli isolates. Overall, 89 (25.9%) isolates had MICs ≥ 32 mg/L to azithromycin, decreasing to 42 (12.2%) when assayed in the presence of Phe-Arg-β-Napthylamide, with 35 of these 42 possessing at least one MRG. Efflux pumps played a role in azithromycin resistance affecting the Minimal Inhibitory Concentration (MIC) levels of 91.2% isolates whereas chromosomal alterations seem to have a minimal role. At least one MRG was found in 22.7% of the isolates with mph(A) being the most commonly found gene. The mph(A) gene plays the main role in the development of azithromycin resistance and 93% of the mph(A)-carrying isolates showed a MIC of 32 mg/L. In the absence of a specific resistance breakpoint our results suggest a MIC of 32 mg/L to be considered in order to detect isolates carrying mechanisms able to confer azithromycin resistance.

Environmental and genetic modulation of the phenotypic expression of antibiotic resistance

Antibiotic resistance can be acquired by mutation or horizontal transfer of a resistance gene, and generally an acquired mechanism results in a predictable increase in phenotypic resistance. However, recent findings suggest that the environment and/or the genetic context can modify the phenotypic expression of specific resistance genes/mutations. An important implication from these findings is that a given genotype does not always result in the expected phenotype. This dissociation of genotype and phenotype has important consequences for clinical bacteriology and for our ability to predict resistance phenotypes from genetics and DNA sequences. A related problem concerns the degree to which the genes/mutations currently identified in vitro can fully explain the in vivo resistance phenotype, or whether there is a significant additional amount of presently unknown mutations/genes (genetic ‘dark matter’) that could contribute to resistance in clinical isolates. Finally, a very important question is whether/how we can identify the genetic features that contribute to making a successful pathogen, and predict why some resistant clones are very successful and spread globally? In this review, we describe different environmental and genetic factors that influence phenotypic expression of antibiotic resistance genes/mutations and how this information is needed to understand why particular resistant clones spread worldwide and to what extent we can use DNA sequences to predict evolutionary success.

Natural Products as Platforms To Overcome Antibiotic Resistance

Natural products have served as powerful therapeutics against pathogenic bacteria since the golden age of antibiotics of the mid-20th century. However, the increasing frequency of antibiotic-resistant infections clearly demonstrates that new antibiotics are critical for modern medicine. Because combinatorial approaches have not yielded effective drugs, we propose that the development of new antibiotics around proven natural scaffolds is the best short-term solution to the rising crisis of antibiotic resistance. We analyze herein synthetic approaches aiming to reengineer natural products into potent antibiotics. Furthermore, we discuss approaches in modulating quorum sensing and biofilm formation as a nonlethal method, as well as narrow-spectrum pathogen-specific antibiotics, which are of interest given new insights into the implications of disrupting the microbiome.

The macrolide antibiotic renaissance

Macrolides represent a large family of protein synthesis inhibitors of great clinical interest due to their applicability to human medicine. Macrolides are composed of a macrocyclic lactone of different ring sizes, to which one or more deoxy-sugar or amino sugar residues are attached. Macrolides act as antibiotics by binding to bacterial 50S ribosomal subunit and interfering with protein synthesis. The high affinity of macrolides for bacterial ribosomes, together with the highly conserved structure of ribosomes across virtually all of the bacterial species, is consistent with their broad-spectrum activity. Since the discovery of the progenitor macrolide, erythromycin, in 1950, many derivatives have been synthesised, leading to compounds with better bioavailability and acid stability and improved pharmacokinetics. These efforts led to the second generation of macrolides, including well-known members such as azithromycin and clarithromycin. Subsequently, in order to address increasing antibiotic resistance, a third generation of macrolides displaying improved activity against many macrolide resistant strains was developed. However, these improvements were accompanied with serious side effects, leading to disappointment and causing many researchers to stop working on macrolide derivatives, assuming that this procedure had reached the end. In contrast, a recent published breakthrough introduced a new chemical platform for synthesis and discovery of a wide range of diverse macrolide antibiotics. This chemical synthesis revolution, in combination with reduction in the side effects, namely, 'Ketek effects', has led to a macrolide renaissance, increasing the hope for novel and safe therapeutic agents to combat serious human infectious diseases.

Resistance to Macrolide Antibiotics in Public Health Pathogens

Macrolide resistance mechanisms can be target-based with a change in a 23S ribosomal RNA (rRNA) residue or a mutation in ribosomal protein L4 or L22 affecting the ribosome's interaction with the antibiotic. Alternatively, mono- or dimethylation of A2058 in domain V of the 23S rRNA by an acquired rRNA methyltransferase, the product of an erm (erythromycin ribosome methylation) gene, can interfere with antibiotic binding. Acquired genes encoding efflux pumps, most predominantly mef(A) + msr(D) in pneumococci/streptococci and msr(A/B) in staphylococci, also mediate resistance. Drug-inactivating mechanisms include phosphorylation of the 2'-hydroxyl of the amino sugar found at position C5 by phosphotransferases and hydrolysis of the macrocyclic lactone by esterases. These acquired genes are regulated by either translation or transcription attenuation, largely because cells are less fit when these genes, especially the rRNA methyltransferases, are highly induced or constitutively expressed. The induction of gene expression is cleverly tied to the mechanism of action of macrolides, relying on antibiotic-bound ribosomes stalled at specific sequences of nascent polypeptides to promote transcription or translation of downstream sequences.

A rapid loop-mediated isothermal amplification (LAMP) method for detection of the macrolide-streptogramin type B resistance gene msrA in Staphylococcus aureus

Macrolide-streptogramin type B resistance (the MSB phenotype) is a multidrug resistance phenotype in Staphylococcus aureus conferred by the resistance gene msrA. However, bacteria having the MSB phenotype are susceptible to lincosamides and 16-membered ring macrolides, which makes profiling resistance genes necessary and urgent for timely and appropriate use of antimicrobials. In this study, the loop-mediated isothermal amplification (LAMP) assay was optimized for prompt detection of the msrA gene. msrA gene sequences were obtained from the National Center for Biotechnology Information (NCBI) database and primers were designed using the LAMP primer designing software PrimerExplorer v.4, which together recognize seven distinct regions of the msrA gene. The specific LAMP primer set designed in this study could amplify the msrA gene within 25min at an isothermal temperature of 62°C. More importantly, the msrA gene could be detected at a sensitivity as low as 100pg. Furthermore, this optimized LAMP assay provided swift detection of the msrA gene even directly from human specimens. In conclusion, this assay may have great clinical application potential for detection of the msrA gene.

Antimicrobial resistance determinants in Acinetobacter baumannii isolates taken from military treatment facilities

Multidrug-resistant (MDR) Acinetobacter baumannii infections are of particular concern within medical treatment facilities, yet the gene assemblages that give rise to this phenotype remain poorly characterized. In this study, we tested 97 clinical A. baumannii isolates collected from military treatment facilities (MTFs) from 2003 to 2009 by using a molecular epidemiological approach that enabled for the simultaneous screening of 236 antimicrobial resistance genes. Overall, 80% of the isolates were found to be MDR, each strain harbored between one and 17 resistant determinants, and a total of 52 unique resistance determinants or gene families were detected which are known to confer resistance to β-lactam (e.g., blaGES-11, blaTEM, blaOXA-58), aminoglycoside (e.g., aphA1, aacC1, armA), macrolide (msrA, msrB), tetracycline [e.g., tet(A), tet(B), tet(39)], phenicol (e.g., cmlA4, catA1, cat4), quaternary amine (qacE, qacEΔ1), streptothricin (sat2), sulfonamide (sul1, sul2), and diaminopyrimidine (dfrA1, dfrA7, dfrA19) antimicrobial compounds. Importantly, 91% of the isolates harbored blaOXA-51-like carbapenemase genes (including six new variants), 40% harbored the blaOXA-23 carbapenemase gene, and 89% contained a variety of aminoglycoside resistance determinants with up to six unique determinants identified per strain. Many of the resistance determinants were found in potentially mobile gene cassettes; 45% and 7% of the isolates contained class 1 and class 2 integrons, respectively. Combined, the results demonstrate a facile approach that supports a more complete understanding of the genetic underpinnings of antimicrobial resistance to better assess the load, transmission, and evolution of MDR in MTF-associated A. baumannii.

Acquired inducible antimicrobial resistance in Gram-positive bacteria

A major contributor to the emergence of antibiotic resistance in Gram-positive bacterial pathogens is the expansion of acquired, inducible genetic elements. Although acquired, inducible antibiotic resistance is not new, the interest in its molecular basis has been accelerated by the widening distribution and often 'silent' spread of the elements responsible, the diagnostic challenges of such resistance and the mounting limitations of available agents to treat Gram-positive infections. Acquired, inducible antibiotic resistance elements belong to the accessory genome of a species and are horizontally acquired by transformation/recombination or through the transfer of mobile DNA elements. The two key, but mechanistically very different, induction mechanisms are: ribosome-sensed induction, characteristic of the macrolide-lincosamide-streptogramin B antibiotics and tetracycline resistance, leading to ribosomal modifications or efflux pump activation; and resistance by cell surface-associated sensing of β-lactams (e.g., oxacillin), glycopeptides (e.g., vancomycin) and the polypeptide bacitracin, leading to drug inactivation or resistance due to cell wall alterations.

Presence of erythromycin and tetracycline resistance genes in lactic acid bacteria from fermented foods of Indian origin

Lactic acid bacteria (LAB) resistant to erythromycin were isolated from different food samples on selective media. The isolates were identified as Enterococcus durans, Enterococcus faecium, Enterococcus lactis, Enterococcus casseliflavus, Lactobacillus salivarius, Lactobacillus reuteri, Lactobacillus plantarum, Lactobacillus fermentum, Pediococcus pentosaceus and Leuconostoc mesenteroides. Of the total 60 isolates, 88 % harbored the ermB gene. The efflux gene msrA was identified in E. faecium, E. durans, E. lactis, E. casseliflavus, P. pentosaceus and L. fermentum. Further analysis of the msrA gene by sequencing suggested its homology to msrC. Resistance to tetracycline due to the genes tetM, tetW, tetO, tetK and tetL, alone or in combination, were identified in Lactobacillus species. The tetracycline efflux genes tetK and tetL occurred in P. pentosaceus and Enterococcus species. Since it appeared that LAB had acquired these genes, fermented foods may be a source of antibiotic resistance.

High frequency of macrolide resistance mechanisms in clinical isolates of Corynebacterium species

The genus Corynebacterium includes a high number of species that are usually isolated from human skin as saprophytes. However, these microorganisms have also been reported as infectious agents in a broad group of patients and have showed broad-spectrum resistance. We studied the susceptibility profiles against macrolides, clindamycin, and streptogramins of 254 clinical strains belonging to the species Corynebacterium urealyticum (120), Corynebacterium amycolatum (66), Corynebacterium jeikeium (17), Corynebacterium striatum (20), Corynebacterium coyleae (12), Corynebacterium aurimucosum (11), and Corynebacterium afermentans subsp. afermentans (8). The MLS(B) phenotype was detected in 186 strains and was associated with the presence of methylase enzymes codified by the erm(X) gene in 171 strains. The erm(B) gene was only detected in two C. urealyticum strains. Fourteen strains showed macrolide resistance, but they did not carry erm genes. mef genes were not detected despite eight C. amycolatum strains showed the M phenotype. Also, the presence of hydrolytic enzymes codified by ere(B) was evaluated, but all results were negative. Resistance to macrolide in Corynebacterium sp. is mainly due to the presence of erm(X) methylase, although other resistance mechanisms could be involved.

Antibiotic resistance in coagulase negative staphylococci isolated from wastewater and drinking water

This study reports the antibiotic resistance patterns of coagulase negative staphylococci (CNS) isolated from a drinking water treatment plant (WTP), a drinking water distribution network, responsible for supplying water to the consumers (WDN), and a wastewater treatment plant (WWTP), responsible for receiving and treating domestic residual effluents. Genotyping and the 16S rRNA gene sequence analysis demonstrated a higher diversity of species both in the WTP (6 species/19 isolates) and WWTP (12 species/47 isolates) than in the WDN (6 species/172 isolates). Staphylococcus pasteuri and Staphylococcus epidermidis prevailed in the WTP and WDN and Staphylococcus saprophyticus in the WWTP. Staphylococci with reduced susceptibility (resistance or intermediary phenotype) to beta-lactams, tetracycline, clindamycin and erythromycin were observed in all types of water and belonged to the three major species groups. The highest resistance rate was found against erythromycin, presumably due to the presence of the efflux pump encoded by the determinant msrA, detected in the majority of the resistant isolates. This study demonstrates that antibiotic resistant CNS may colonize different types of water, namely drinking water fulfilling all the quality standards.

Update on macrolide-lincosamide-streptogramin, ketolide, and oxazolidinone resistance genes

This Minireview summarizes the changes in the field of bacterial resistance to macrolide, lincosamide, streptogramin, ketolide, and oxazolidinone (MLSKO) antibiotics since the nomenclature review in 1999. A total of 66 genes conferring resistance to this group of antibiotics has now been identified and includes 13 new rRNA methylase genes, four ATP-binding transporter genes coding for efflux proteins, and five new inactivating enzymes. During this same time period, 73 new genera carrying known rRNA methylase genes and 87 new genera carrying known efflux and/or inactivating genes have been recognized. The number of bacteria with mutations in the genes for 23S rRNA, L4 and L22 ribosomal proteins, resulting in reduced susceptibility to some members of the group of MLSKO antibiotics has also increased and now includes nine different Gram-positive and 10 different Gram-negative genera. New conjugative transposons carrying different MLSKO genes along with an increased number of antibiotics and/or heavy metal resistance genes have been identified. These mobile elements may play a role in the continued spread of the MLSKO resistance genes into new species, genera, and ecosystems.

Active Export Proteins Mediating Drug Resistance in Staphylococci

Drug resistance mediated by integral membrane transporters is an important mode of cellular resistance to cytotoxic agents across all classes of living organisms. Gram-positive bacteria, such as staphylococcal species, are not encapsulated by a selective outer membrane permeability barrier. Therefore, these organisms often employ integral membrane drug transport systems to maintain cellular concentrations of antimicrobials at subtoxic levels. Staphylococcal species, including the opportunistic human pathogen Staphylococcus aureus, encode a multitude of drug exporters, encompassing transporters from each of the five currently recognized families of bacterial drug resistance transporters. A number of these transporters are chromosomally encoded and allow the host cell to realize clinically significant levels of drug resistance after minor mutations to regulatory regions. Others are plasmid-encoded and can be easily passed between staphylococcal strains and species, or acquired from other Gram-positive genera. In combination, staphylococcal drug transporters potentiate resistance to a vast array of antimicrobial compounds, including macrolide, quinolone, tetracycline and streptogramin antibiotics, as well as a broad range of biocides, such as quaternary ammonium compounds, biguanidines and diamidines. An understanding of the genetic and molecular properties of drug transporters will lead to effective treatments of staphylococcal infections. Here we provide a detailed review of the active drug transporters of the staphylococci.

Efflux pumps as antimicrobial resistance mechanisms

Antibiotic resistance continues to hamper antimicrobial chemotherapy of infectious disease, and while biocide resistance outside of the laboratory is as yet unrealized, in vitro and in vivo episodes of reduced biocide susceptibility are not uncommon. Efflux mechanisms, both drug-specific and multidrug, are important determinants of intrinsic and/or acquired resistance to these antimicrobials in important human pathogens. Multidrug efflux mechanisms are generally chromosome-encoded, with their expression typically resultant from mutations in regulatory genes, while drug-specific efflux mechanisms are encoded by mobile genetic elements whose acquisition is sufficient for resistance. While it has been suggested that drug-specific efflux systems originated from efflux determinants of self-protection in antibiotic-producing Actinomycetes, chromosomal multidrug efflux determinants, at least in Gram-negative bacteria, are appreciated as having an intended housekeeping function unrelated to drug export and resistance. Thus, it will be important to elucidate the intended natural function of these efflux mechanisms in order, for example, to anticipate environmental conditions or circumstances that might promote their expression and, so, compromise antimicrobial chemotherapy. Given the clinical significance of antimicrobial exporters, it is clear that efflux must be considered in formulating strategies for treatment of drug-resistant infections, both in the development of new agents, for example, less impacted by efflux or in targeting efflux directly with efflux inhibitors.