Multidrug-Resistant Tuberculosis

Abandono ou descontinuidade do tratamento da tuberculose em Rio Branco, Acre

Esta pesquisa investiga os fatores que estão relacionados à descontinuidade do tratamento da tuberculose em Rio Branco-Acre e foi realizada com uma contribuição da etnografia para uma apreensão da realidade a ser estudada. Foi desenvolvida em duas etapas de mapeamento: na primeira, procedeu-se à busca de informações na base de dados do SINAN (Sistema de Notificação de Agravos Nacional) e na Coordenação Municipal do Programa de Tuberculose, a segunda etapa procurou registrar fatos a partir da observação do atendimento numa unidade de saúde que presta assistência aos portadores de tuberculose em tratamento, e entrevistas com eles, assim como de profissionais que prestavam assistência. Na análise das observações, coletas de narrativas e entrevistas, observou-se que os profissionais dos serviços de saúde caracterizam algumas pessoas como propícias a descontinuarem o tratamento e não consideram os diferentes modos de vida na abordagem de seus pacientes, dificultando, assim, a formação do vínculo e favorecendo a descontinuidade ou o abandono. Identificou-se também que as pessoas que faziam o tratamento da tuberculose tinham várias formas de lidar com as limitações que foram geradas pela doença, como a restrição para o trabalho, entre outras, e as pessoas que o descontinuaram levavam em consideração seu sistema de crenças e valores, bem como a própria percepção de saúde/doença, devido a estarem se sentindo curados quando o interromperam.

Multi-drug-resistant tuberculosis: the experience of an urban tertiary care hospital in South India using automated BACTEC 460 TB

The emergence of multi-drug-resistant (MDR) strains has been a major obstacle in the tuberculosis (TB) control programme. In the present study we looked into the prevalence of MDR-TB in an urban tertiary care hospital in South India over four years (2007-2010). During this period, 641 clinical specimens (317 respiratory specimens and 324 non-respiratory specimens) were received for mycobacteriological culture and drug susceptibility testing for first-line drugs, using the BACTEC 460 TB system. Mycobacterium tuberculosis (MTB) was isolated in 34.8% (n = 223) specimens. Of the total 223 MTB isolates 83 (37.2%) were MDR. Forty-two percent of the pulmonary MTB isolates (n = 72) and 20.4% of the extra-pulmonary isolates (n = 10) were MDR. Although we observed a high percentage of drug resistance, the prevalence of MDR was not observed to vary significantly within the four years which suggested good management.

Potent inhibitors of a shikimate pathway enzyme from Mycobacterium tuberculosis: combining mechanism- and modeling-based design

Tuberculosis remains a serious global health threat, with the emergence of multidrug-resistant strains highlighting the urgent need for novel antituberculosis drugs. The enzyme 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase (DAH7PS) catalyzes the first step of the shikimate pathway for the biosynthesis of aromatic compounds. This pathway has been shown to be essential in Mycobacterium tuberculosis, the pathogen responsible for tuberculosis. DAH7PS catalyzes a condensation reaction between P-enolpyruvate and erythrose 4-phosphate to give 3-deoxy-D-arabino-heptulosonate 7-phosphate. The enzyme reaction mechanism is proposed to include a tetrahedral intermediate, which is formed by attack of an active site water on the central carbon of P-enolpyruvate during the course of the reaction. Molecular modeling of this intermediate into the active site reported in this study shows a configurational preference consistent with water attack from the re face of P-enolpyruvate. Based on this model, we designed and synthesized an inhibitor of DAH7PS that mimics this reaction intermediate. Both enantiomers of this intermediate mimic were potent inhibitors of M. tuberculosis DAH7PS, with inhibitory constants in the nanomolar range. The crystal structure of the DAH7PS-inhibitor complex was solved to 2.35 Å. Both the position of the inhibitor and the conformational changes of active site residues observed in this structure correspond closely to the predictions from the intermediate modeling. This structure also identifies a water molecule that is located in the appropriate position to attack the re face of P-enolpyruvate during the course of the reaction, allowing the catalytic mechanism for this enzyme to be clearly defined.

Rifampin stability in 7H9 broth and Löwenstein-Jensen medium

The objectives of this study were to monitor the stability of rifampin (RIF) in Löwenstein-Jensen medium (L-J medium) and 7H9 broth, which are the media commonly used for drug susceptibility testing (DST) of Mycobacterium tuberculosis. Rifampin degradation in stock solution, 7H9 broth, and L-J medium and during the inspissation process for L-J medium preparation was serially monitored by high-performance liquid chromatography (HPLC). L-J medium-based DST was conducted to examine the effect of L-J medium storage on the DST outcome. The RIF stock solution was stable for at least 3 months when kept at either 4°C or -20°C; RIF in 7H9 broth and L-J medium was almost 50% decayed after 1 week of storage at 37°C, and rifampin could not be detected in 7H9 or L-J medium after 3 weeks or 6 weeks of storage at 37°C. Approximately half of the drug was decomposed after 4 months of storage at 4°C for both media, and after 6 months of storage at 4°C, RIF in L-J medium was undetectable, while 38% of RIF remained in 7H9 medium. Approximately 21, 24, 29, and 35% RIF degradations were detected when the L-J medium was coagulated at 75°C, 80°C, 85°C, and 90°C, respectively. The DST outcomes when using L-J medium stored for different periods of time were consistent with the RIF concentration monitoring data. Rifampin in stock solution is stable for at least 3 months at a reduced storage temperature. Media containing RIF should be prepared strictly according to validated standard operating procedures. RIF degradation is a possible reason for false resistance categorizations of M. tuberculosis isolates in the clinical laboratory.

Mycobacterium tuberculosis phosphoribosylpyrophosphate synthetase: biochemical features of a crucial enzyme for mycobacterial cell wall biosynthesis

The selection and soaring spread of Mycobacterium tuberculosis multidrug-resistant (MDR-TB) and extensively drug-resistant strains (XDR-TB) is a severe public health problem. Currently, there is an urgent need for new drugs for tuberculosis treatment, with novel mechanisms of action and, moreover, the necessity to identify new drug targets. Mycobacterial phosphoribosylpyrophosphate synthetase (MtbPRPPase) is a crucial enzyme involved in the biosynthesis of decaprenylphosphoryl-arabinose, an essential precursor for the mycobacterial cell wall biosynthesis. Moreover, phosphoribosylpyrophosphate, which is the product of the PRPPase catalyzed reaction, is the precursor for the biosynthesis of nucleotides and of some amino acids such as histidine and tryptophan. In this context, the elucidation of the molecular and functional features of MtbPRPPase is mandatory. MtbPRPPase was obtained as a recombinant form, purified to homogeneity and characterized. According to its hexameric form, substrate specificity and requirement of phosphate for activity, the enzyme proved to belong to the class I of PRPPases. Although the sulfate mimicked the phosphate, it was less effective and required higher concentrations for the enzyme activation. MtbPRPPase showed hyperbolic response to ribose 5-phosphate, but sigmoidal behaviour towards Mg-ATP. The enzyme resulted to be allosterically activated by Mg²⁺ or Mn²⁺ and inhibited by Ca²⁺ and Cu²⁺ but, differently from other characterized PRPPases, it showed a better affinity for the Mn²⁺ and Cu²⁺ ions, indicating a different cation binding site geometry. Moreover, the enzyme from M. tuberculosis was allosterically inhibited by ADP, but less sensitive to inhibition by GDP. The characterization of M. tuberculosis PRPPase provides the starting point for the development of inhibitors for antitubercular drug design.

CD46 engagement on human CD4+ T cells produces T regulatory type 1-like regulation of antimycobacterial T cell responses

Understanding the regulation of human immune responses is critical for vaccine development and treating infectious diseases. We have previously shown that simultaneous engagement of the T cell receptor (TCR) and complement regulator CD46 on human CD4(+) T cells in the presence of interleukin-2 (IL-2) induces potent secretion of the immunomodulatory cytokine IL-10. These T cells mediate IL-10-dependent suppression of bystander CD4(+) T cells activated in vitro with anti-CD3 and anti-CD28 costimulation, reflecting a T regulatory type 1 (Tr1)-like phenotype. However, CD46-mediated negative regulation of pathogen-specific T cells has not been described. Therefore, we studied the ability of CD46-activated human CD4(+) T cells to suppress T cell responses to Mycobacterium bovis BCG, the live vaccine that provides infants protection against the major human pathogen Mycobacterium tuberculosis. Our results demonstrate that soluble factors secreted by CD46-activated human CD4(+) T cells suppress mycobacterium-specific CD4(+), CD8(+), and γ(9)δ(2) TCR(+) T cells. Dendritic cell functions were not downregulated in our experiments, indicating that CD46-triggered factors directly suppress pathogen-specific T cells. Interestingly, IL-10 appeared to play a less pronounced role in our system, especially in the suppression of γ(9)δ(2) TCR(+) T cells, suggesting the presence of additional undiscovered soluble immunoregulatory factors. Blocking endogenous CD46 signaling 3 days after mycobacterial infection enhanced BCG-specific T cell responses in a subset of volunteers. Taken together, these results indicate that CD46-dependent negative regulatory mechanisms can impair T cell responses vital for immune defense against mycobacteria. Therefore, modulating CD46-induced immune regulation could be integral to the development of improved tuberculosis therapeutics or vaccines.

HIV-tuberculosis coinfection in Southern California: evaluating disparities in disease burden

OBJECTIVES

We sought to understand tuberculosis (TB) and HIV coinfection trends in San Diego County, California, and to identify associations between sociodemographic risk factors and TB and HIV coinfection.

METHODS

We analyzed TB surveillance data from 1993 through 2007. TB cases were grouped by HIV status: positive, negative, or unknown. We used Poisson regression to estimate trends and tested associations between TB and HIV coinfection and sociodemographic risk factors with polychotomous logistic regression.

RESULTS

Of 5172 TB cases, 8.8% were also infected with HIV. Incidence of coinfected cases did not change significantly over the period studied, but the proportion of cases among Hispanics increased significantly, whereas cases among non-Hispanic Whites and Blacks decreased. TB cases with HIV coinfection were significantly more likely to be Hispanic, male, injection drugs users, and aged 30 to 49 years, relative to cases with TB disease only.

CONCLUSIONS

The burden of TB and HIV in San Diego has shifted to Hispanics in the last decade. To address this health disparity, binational TB and HIV prevention efforts are needed.

Multidrug-resistant tuberculosis of the spine--is it the beginning of the end? A study of twenty-five culture proven multidrug-resistant tuberculosis spine patients

STUDY DESIGN

Prospective cohort study.

OBJECTIVE

We report the first study of multidrug-resistant tuberculosis (MDR-TB) in the spine. The aim was to determine the clinical, radiologic, and drug resistance profile as well as the factors associated with treatment outcome of MDR-TB in the spine.

SUMMARY OF BACKGROUND DATA

Tuberculosis of the spine is the most common extrapulmonary form of tuberculosis in the Asian subcontinent. The disease in few cases is resistant to the primary anti-Koch's medications and the number of cases detected is on the rise. Multidrug resistant form of tuberculosis of the spine is ill reported in the literature. The diagnosis, management thus remains a challenge to the treating surgeon. This study tries to assess these critical issues of this "new" disease.

METHODS

Described here are the clinical characteristics of 25 MDR-TB spine patients identified in the study and their drug susceptibility patterns. They were followed up clinically, radiologically after a biopsy, culture, and Drug Susceptibility Testing. According to their Drug Susceptibility Testing pattern and previous history of Anti-Tubercular Treatment (ATT), individualized treatment regimens were tailored for each patient by an expert physician.

RESULTS

Majority of the patients seen in the present study were in the productive years of their life. (Males (9) mean age: 38.5 years and females (16) mean age: 34.3 years. Four patients were defaulters of the ATT. The average number of drugs used was 6, including 4 second line drugs. Average treatment duration was 24 months. Almost 50% of the patients had adverse drug effects. Of the 25 patients, 19 achieved healed status and 6 are still on treatment. Four patients required surgery for mechanical instability of the spine. Radiologic improvement was observed in all the cases after a mean treatment of 6 months. Five predictors were identified for successful outcome of MDR-TB. They include progressive clinical improvement at 6 months, radiologic improvement during treatment and disease with Mycobacterium tuberculosis strains exhibiting resistance to less than or up to 3 antitubercular drugs, use of less than or up to 4 second-line drugs in treatment, and no change of regimen during treatment.

CONCLUSION

MDR-TB of the spine is a different disease and is here to stay. There is an urgent need to include culture and drug susceptibility testing in the protocol for the treatment of tuberculosis of the spine.

Treatment outcomes of multidrug-resistant tuberculosis: a systematic review and meta-analysis

Background

Treatment outcomes for multidrug-resistant Mycobacterium Tuberculosis (MDRTB) are generally poor compared to drug sensitive disease. We sought to estimate treatment outcomes and identify risk factors associated with poor outcomes in patients with MDRTB.

Methodology/Principal Findings

We performed a systematic search (to December 2008) to identify trials describing outcomes of patients treated for MDRTB. We pooled appropriate data to estimate WHO-defined outcomes at the end of treatment and follow-up. Where appropriate, pooled covariates were analyzed to identify factors associated with worse outcomes. Among articles identified, 36 met our inclusion criteria, representing 31 treatment programmes from 21 countries. In a pooled analysis, 62% [95% CI 57–67] of patients had successful outcomes, while 13% [9]–[17] defaulted, 11% [9]–[13] died, and 2% [1]–[4] were transferred out. Factors associated with worse outcome included male gender 0.61 (OR for successful outcome) [0.46–0.82], alcohol abuse 0.49 [0.39–0.63], low BMI 0.41[0.23–0.72], smear positivity at diagnosis 0.53 [0.31–0.91], fluoroquinolone resistance 0.45 [0.22–0.91] and the presence of an XDR resistance pattern 0.57 [0.41–0.80]. Factors associated with successful outcome were surgical intervention 1.91 [1.44–2.53], no previous treatment 1.42 [1.05–1.94], and fluoroquinolone use 2.20 [1.19–4.09].

Conclusions/Significance

We have identified several factors associated with poor outcomes where interventions may be targeted. In addition, we have identified high rates of default, which likely contributes to the development and spread of MDRTB.

Chemical synthesis of all phosphatidylinositol mannoside (PIM) glycans from Mycobacterium tuberculosis

The emergence of multidrug-resistant tuberculosis (TB) and problems with the BCG tuberculosis vaccine to protect humans against TB have prompted investigations into alternative approaches to combat this disease by exploring novel bacterial drug targets and vaccines. Phosphatidylinositol mannosides (PIMs) are biologically important glycoconjugates and represent common essential precursors of more complex mycobacterial cell wall glycolipids including lipomannan (LM), lipoarabinomannan (LAM), and mannan capped lipoarabinomannan (ManLAM). Synthetic PIMs constitute important biochemical tools to elucidate the biosynthesis of this class of molecules, to reveal PIM interactions with host cells, and to investigate the function of PIMs as potential antigens and/or adjuvants for vaccine development. Here, we report the efficient synthesis of all PIMs including phosphatidylinositol (PI) and phosphatidylinositol mono- to hexa-mannoside (PIM1 to PIM6). Robust synthetic protocols were developed for utilizing bicyclic and tricyclic orthoesters as well as mannosyl phosphates as glycosylating agents. Each synthetic PIM was equipped with a thiol-linker for immobilization on surfaces and carrier proteins for biological and immunological studies. The synthetic PIMs were immobilized on microarray slides to elucidate differences in binding to the dendritic cell specific intercellular adhesion molecule-grabbing nonintegrin (DC-SIGN) receptor. Synthetic PIMs served as immune stimulators during immunization experiments in C57BL/6 mice when coupled to the model antigen keyhole-limpet hemocyanin (KLH).

Trends in Mycobacterium tuberculosis resistance, Pakistan, 1990-2007

OBJECTIVE

To determine resistance trends including the emergence of extensive drug resistance (XDR) in Mycobacterium tuberculosis (MTB) isolates from Pakistan.

METHODS

This was a retrospective analysis of MTB isolates (January 1990-June 2007) collected from the four provinces of Pakistan. Standard methods were used to isolate MTB. Susceptibility against rifampin, isoniazid, streptomycin, ethambutol, ethionamide, capreomycin, cycloserine, and ciprofloxacin was tested using the agar proportion method, while susceptibility to pyrazinamide was determined using BACTEC. Resistance over the study period was assessed using Chi-square for trend analysis.

RESULTS

Resistance in 15343 MTB isolates showed a steady increase over the study period; a significant trend of increasing resistance was noted in three of the four provinces included in the study. XDR-tuberculosis was first seen in 1998; to date 22 XDR cases have been documented. MTB resistance was significantly lower (p<0.001) in patients under 5 and over 60 years of age, and significantly higher in males than females (p<0.001). MTB resistance was also higher in pulmonary than extrapulmonary isolates (p<0.001).

CONCLUSION

This study documenting a steady increase in resistance among MTB isolates and the emergence of XDR strains is concerning. The data highlight the need to ensure adequate treatment of cases in order to prevent a further increase in multidrug resistance within the country. The importance of developing systems for monitoring drug resistance through nationwide surveillance is emphasized.

Differential cytokine secretion and early treatment response in patients with pulmonary tuberculosis

Biomarkers for treatment response would facilitate the testing of urgently needed new anti-tuberculous drugs. The present study investigated the profiles of 30 proinflammatory, anti-inflammatory and angiogenic factors [epidermal growth factor, eotaxin, fractalkine, granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor, interleukin (IL)-1alpha, IL-1beta, IL-1ra, IL-2, IL-4, IL-5, IL-6, IL-7, IL-8, IL-10, IL-12p40, IL-12p70, IL-13, IL-15, IL-17, interferon-gamma, interferon-inducible protein-10, Krebs von den Lungen-6, monocyte chemotactic protein-1, macrophage inflammatory protein (MIP)-1alpha, MIP-1beta, sCD40L, transforming growth factor-alpha, tumour necrosis factor-alpha and vascular endothelial growth factor] in the plasma of 12 healthy tuberculin skin test-positive community controls and 20 human immunodeficiency virus-negative patients with active tuberculosis (TB) and identified potential biomarkers for early treatment response. We showed differences in the level of circulating cytokines between healthy controls and TB patients, but also between fast responders and slow responders to anti-tuberculosis treatment. The general discriminant analysis based on pre-treatment and week 1 measurements identified 10 sets of three-variable models that could classify fast and slow responders with up to 83% accuracy. Overall, this study shows the potential of cytokines as indicators of anti-tuberculosis treatment response.

Mutations at embB codon 306 are an important molecular indicator of ethambutol resistance in Mycobacterium tuberculosis

Ethambutol resistance in clinical Mycobacterium tuberculosis isolates is associated primarily with missense mutations in the embB gene. However, recent reports have described the presence of embB mutations, especially those at embB codon 306, in isolates susceptible to ethambutol. To clarify the role of embB mutations in ethambutol resistance, we sequenced the ethambutol resistance-determining region in spontaneous ethambutol-resistant mutants. In our study, 66% of spontaneous mutants contained a single point mutation in embB, with 55% of these occurring at embB 306. The MIC of ethambutol for spontaneous mutants was increased two- to eightfold relative to the pansusceptible M. tuberculosis strains from which the mutants were generated. To further characterize the role of embB 306 mutations, we directly introduced mutant alleles, embB(M306V) or embB(M306I), into pansusceptible M. tuberculosis strains and conversely reverted mutant alleles in spontaneous ethambutol-resistant mutants back to those of the wild type via allelic exchange using specialized linkage transduction. We determined that the MIC of ethambutol was reduced fourfold for three of the four spontaneous ethambutol-resistant embB 306 mutants when the mutant allele was replaced with the wild-type embB allele. The MIC for one of the spontaneous mutants genetically reverted to wild-type embB was reduced by only twofold. When the wild-type embB allele was converted to the mutant allele embB(M306V), the ethambutol MIC was increased fourfold, and when the allele was changed to M306I, the ethambutol MIC increased twofold. Our data indicate that embB 306 mutations are sufficient to confer ethambutol resistance, and detection of these mutations should be considered in the development of rapid molecular tests.

Case series report of a linezolid-containing regimen for extensively drug-resistant tuberculosis

OBJECTIVE

To determine whether linezolid is safe and well tolerated in the treatment of extensively drug-resistant tuberculosis (XDR-TB).

MATERIALS AND METHODS

The was conducted in a specialized tuberculosis ward for multidrug-resistant tuberculosis (MDR-TB) on the Chest Service of Bellevue Hospital Center, which is a 768-bed public hospital in New York City. Seven patients with confirmed MDR-TB or XDR-TB who were still culture positive despite appropriate directly observed therapy were treated with a regimen containing linezolid and at least one other active agent.

RESULTS

The linezolid-containing regimen led to sustained negative conversion of sputum cultures and radiographic improvement in all patients. Long-term therapy (longest duration of therapy, 28 months) was well tolerated in most patients. Neutropenia developed in three patients, but was reversible, and peripheral neuropathy developed in two patients.

CONCLUSIONS

Linezolid remains a promising possible addition to our therapeutic armamentarium against XDR-TB. Linezolid is associated with side effects that can be adequately managed. Further studies to define the mechanism of action and optimum dose should be performed.

Tuberculosis among dislocated North Koreans entering Republic of Korea since 1999

The collapse of North Korea's public health system has increased the development of tuberculosis (TB) in its populace. This study investigated the prevalence of active and latent TB infection (LTBI) in such people who have settled in the Republic of Korea since 1999. From 1999 to August 2006, 7,722 dislocated North Koreans entered the Republic of Korea and all were screened immediately for active TB. Demographic and clinical characteristics were reviewed from the official records of the Settlement Support Office for Dislocated North Koreans, based in the Ministry of Unification. Of 7,722 participants, 87 (1.13%) were diagnosed with active TB from 1999 to August 2006. Of these, 78 (90%) had pulmonary TB. Checking for the presence of a Bacille Calmette-Guerin (BCG) scar and tuberculin skin test has been performed in all dislocated North Koreans since November 2005. Of 1,112 participants, BCG vaccination scars were found in 67.4%. The tuberculin-positive rate using two tuberculin unit doses of the purified protein derivative RT23 (> or =10mm in diameter) was 81.5%. The prevalence of active TB and LTBI in dislocated North Koreans was high. Because this group bears a disproportionate burden of TB, we need to initiate a specific control programme and to plan for the impact of this disease in the Republic of Korea.

Audiologic monitoring of multi-drug resistant tuberculosis patients on aminoglycoside treatment with long term follow-up

BACKGROUND

Multi-drug resistant tuberculosis has emerged as a significant problem with the resurfacing of tuberculosis and thus the need to use the second line drugs with the resultant increased incidence of adverse effects. We discuss the effect of second line aminoglycoside anti-tubercular drugs on the hearing status of MDR-TB patients.

METHODS

Sixty four patients were put on second line aminoglycoside anti-TB drugs. These were divided into three groups: group I, 34 patients using amikacin, group II, 26 patients using kanamycin and group III, 4 patients using capreomycin.

RESULTS

Of these, 18.75% of the patients developed sensorineural hearing loss involving higher frequencies while 6.25% had involvement of speech frequencies also. All patients were seen again approximately one year after aminoglycoside discontinuation and all hearing losses were permanent with no threshold improvement.

CONCLUSION

Aminoglycosides used in MDR-TB patients may result in irreversible hearing loss involving higher frequencies and can become a hearing handicap as speech frequencies are also involved in some of the patients thus underlining the need for regular audiologic evaluation in patients of MDR-TB during the treatment.

Diagnosis and treatment of tuberculous pleural effusion in 2006

Tuberculous (TB) pleural effusion occurs in approximately 5% of patients with Mycobacterium tuberculosis infection. The HIV pandemic has been associated with a doubling of the incidence of extrapulmonary TB, which has resulted in increased recognition of TB pleural effusions even in developed nations. Recent studies have provided insights into the immunopathogenesis of pleural TB, including memory T-cell homing and chemokine activation. The definitive diagnosis of TB pleural effusions depends on the demonstration of acid-fast bacilli in the sputum, pleural fluid, or pleural biopsy specimens. The diagnosis can be established in a majority of patients from the clinical features, pleural fluid examination, including cytology, biochemistry, and bacteriology, and pleural biopsy. Measurement of adenosine deaminase and interferon-gamma in the pleural fluid and polymerase chain reaction for M tuberculosis has gained wide acceptance in the diagnosis of TB pleural effusions. Although promising, these tests require further evaluation before their routine use can be recommended. The treatment of TB pleural effusions in patients with HIV/AIDS is essentially similar to that in HIV-negative patients. At present, evidence regarding the use of corticosteroids in the treatment of TB pleural effusion is not clear-cut.

Molecular Mechanisms of Antibacterial Multidrug Resistance

Treatment of infections is compromised worldwide by the emergence of bacteria that are resistant to multiple antibiotics. Although classically attributed to chromosomal mutations, resistance is most commonly associated with extrachromosomal elements acquired from other bacteria in the environment. These include different types of mobile DNA segments, such as plasmids, transposons, and integrons. However, intrinsic mechanisms not commonly specified by mobile elements-such as efflux pumps that expel multiple kinds of antibiotics-are now recognized as major contributors to multidrug resistance in bacteria. Once established, multidrug-resistant organisms persist and spread worldwide, causing clinical failures in the treatment of infections and public health crises.