Tuesday, February 26, 2008
Extensively drug-resistant tuberculosis
Extensively drug-resistant tuberculosis
Dtsch Med Wochenschr. 2008 Feb
C. Lange1, M. P. Grobusch2, D. Wagner3
1 Klinische Infektiologie, Medizinische Klinik, Forschungszentrum Borstel2 Infectious Diseases Unit, Division of Clinical Microbiology and Infectious Diseases, National Health Laboratory Services and School of Pathology, Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, Südafrika3 Zentrum für Infektiologie und Reisemedizin, Medizinische Klinik, Universität Freiburg
Recently an increasing number of antibiotic-resistant MYCOBACTERIUM TUBERCULOSIS (MTB) strains have been described worldwide. The term XDR- (extensively drug-resistant) tuberculosis (TB) has been introduced by the World Health Organisation (WHO) to characterize multi-drug-resistant MTB strains that are in vitro resistant against fluorochinolones and one of the injectible substances amikacin, capreomycin or kanamycin in addition to isoniazid and rifampin. Strains of XDR-MTB are currently increasingly seen in HIV-seropositive individuals with tuberculosis in southern Africa, where these strains are passed by person-to person contact. XDR-TB has become a serious problem for the health administrations in this region. In contrast, cases of XDR-TB are only rarely seen in Germany so far, mainly among the population of pre-treated migrants from eastern Europe. The development of rapid diagnostic tests for resistance testing and new drugs for the treatment of tuberculosis has lacked support for several decades. The sudden emergence of XDR-MTB strains now warrants immediate action for the development of such tests and new classes of antibiotics to give all patients with TB a chance for a successful treatment.
Saturday, February 23, 2008
Detection and inhibition of bacterial cell-cell communication
Rice SA, McDougald D, Givskov M, Kjelleberg S.
Bacteria communicate with other members of their community through the secretion and perception of small chemical cues or signals. The recognition of a signal normally leads to the expression of a large suite of genes, which in some bacteria are involved in the regulation of virulence factors, and as a result, these signaling compounds are key regulatory factors in many disease processes. Thus, it is of interest when studying pathogens to understand the mechanisms used to control the expression of virulence genes so that strategies might be devised for the control of those pathogens. Clearly, the ability to interfere with this process of signaling represents a novel approach for the treatment of bacterial infections. There is a broad range of compounds that bacteria can use for signaling purposes, including fatty acids, peptides, N-acylated homoserine lactones, and the signals collectively called autoinducer 2 (AI-2). This chapter will focus on the latter two signaling systems as they are present in a range of medically relevant bacteria, and here we describe assays for determining whether an organism produces a particular signal and assays that can be used to identify inhibitors of the signaling cascade. Lastly, the signal detection and inhibition assays will be directly linked to the expression of virulence factors of specific pathogens.
PMID: 18287747 [PubMed - in process]
Friday, February 22, 2008
Pseudomonas aeruginosa Serious Infections: Mono or Combination Antimicrobial Therapy?
Curr Med Chem. 2008
Bassetti M, Righi E, Viscoli C.
Infectious Diseases Division, San Martino University Hospital, Genoa, Italy. firstname.lastname@example.org.
P. aeruginosa is a serious cause of infection with reported rates of mortality being up to 61%. Several studies evidenced a correlation between hospital mortality due to P. aeruginosa bloodstream infections and an inappropriate antimicrobial treatment. Increasing resistance in P. aeruginosa isolates complicates the selection of adequate empirical therapy in severe infections and P. aeruginosa is often indistinguishable from other gram-negative bacterial infections. For these reasons, present guidelines for the treatment of suspected P. aeruginosa bacteraemia recommend the rapid introduction of empirical antimicrobial therapy that includes at least one antipseudomonal agent until having microbiological results. Current consensus favours the use of empirical combination, balancing the potential for greater toxicity against the lower emergence of antimicrobial resistance and the greater killing that might be achieved by combination therapies acting synergistically. Advantages and disadvantages of combination therapy towards monotherapy for P. aeruginosa severe infections, current antibiotics used for P. aeruginosa severe infections and main studies published on this issue are reviewed.
PMID: 18289007 [PubMed - in process]
Pseudomonas aeruginosa: combined treatment vs. monotherapy
Med Intensiva. 2007 Mar
Bodí M, Garnacho J.
Hospital Universitario Joan XXIII de Tarragona, Tarragona, España. email@example.com
Keywords: Pseudomonas aeruginosa, combination therapy, microbiological results.
Pseudomonas aeruginosa is a pathogen commonly encountered in clinical practice in critically ill patients. It is a serious cause of infection, associated with a high rate of morbidity and mortality. Inappropriate antimicrobial therapy and delay in starting effective antimicrobial therapy is associated with worse prognostic. This microorganism is clinically indistinguishable from others forms of gram-negative bacterial infection. The rate of multidrug-resistant P. aeruginosa has increased in the last years. For these reasons, patients with Pseudomonas infection might receive empirical antibiotics that are inactive against Pseudomonas, especially before antibiotic susceptibility results become available. It remains controversial whether combination therapy, given empirically or as definitive treatment, for suspected Pseudomonas aeruginosa infections is justifiable. In the present article, we aimed to review recent studies that have evaluated the impact of combination therapy on Pseudomonas infections outcome and we exhibit our point of view in this subject. It seems justifiable to start combination therapy with two antipseudomonal agents in patients with risk for Pseudomonas infection during the first 3-5 days, until having microbiological results. This combination therapy must be changed to monotherapy on the basis on the specific susceptibility pattern of the initial isolate. In cases without microbiological diagnosis and poor outcome, combination therapy will be maintained and other causes of infection will be studied. Multicentre prospective randomized trials in critically ill patients are needed to determine which antimicrobials combinations improve outcome in Pseudomonas infections.
Pseudomonas aeruginosa bloodstream infections: how should we treat them?
Int J Antimicrob Agents. 2007 N
Pseudomonas aeruginosa remains a major cause of bloodstream infections associated with high mortality. Adequacy of empirical therapy seems to influence outcome. Because of its high intrinsic resistance and its capacity to develop resistance during therapy, exposure to antimicrobial therapies frequently leads to subsequent P. aeruginosa bacteraemia with resistant isolates, increasing the risk of inadequate empirical therapy. Therefore empirical therapy should not include antimicrobial agents used in the last few months. Definitive combination therapy does not influence the prognosis of P. aeruginosa bacteraemia. Similarly, empirical combination therapy does not improve survival until receipt of the antibiogram. In contrast, empirical combination therapy does improve survival from the day of receipt of antibiogram to day 30. We therefore suggest that patients suspected of P. aeruginosa bacteraemia should receive empirical combination therapy until receipt of the antibiogram, followed by downgrading to an adequate monotherapy. This strategy might reduce mortality in P. aeruginosa bloodstream infections without exposing the patient to an excessive risk of adverse events. Antimicrobial therapies might select P. aeruginosa isolates with particular virulence phenotypes. The influence of specific virulence determinants on the prognosis of P. aeruginosa bacteraemia, as well as the potential benefit of virulence inhibition, deserves further investigation.
Monday, February 18, 2008
Experts warn of potential dengue fever outbreak in the United States
David Morens, MD, and Anthony Fauci, MD, caution that dengue fever could pose a significant threat to the continental United States.
by Kirsten H. EllisIDN Staff Writer
Dengue fever and its severe forms, dengue hemorrhagic fever and dengue shock syndrome, are potential threats to residents of the continental United States, according to a recent commentary by leading health experts.
Public health officials need to be aware of the threat because no treatments or vaccines for dengue are currently available. Dengue fever is already present along the U.S.-Mexico border and in Puerto Rico.
“Clinicians should know how to identify and diagnose dengue fever. They need to be aware of dengue fever and where it can occur, which is theoretically anywhere in the United States for imported cases,” David Morens, MD, National Institute of Allergy and Infectious Diseases senior advisor, told Infectious Disease News. “Dengue fever was a common occurrence in the past and could be again where mosquito vectors are present, particularly in urban areas.”
Morens and Anthony Fauci, MD, director of National Institute of Allergy and Infectious Diseases, wrote about the threat of dengue fever in the United States in a recent edition of the Journal of the American Medical Association.
Reemergence of dengue fever
“In recent decades, dengue disease has followed vector expansion and dengue hemorrhagic fever, a deadly but poorly understood complication, has tended to follow,” Morens and Fauci wrote in their commentary. “The combined effects of global urbanization and increasing air travel are expected to make dengue fever a growing international health problem for the foreseeable future.”
Dengue fever is considered among the most important reemerging infectious diseases with an estimated 50 million to 100 million cases annually. WHO case estimates include 500,000 hospitalizations and 22,000 deaths each year, most of which occur in children.
In their commentary, Morens and Fauci outlined the background of dengue fever and the adaption and movement of its vector, Aedes aegypti, which already is established in much of the tropical and subtropical world. An alternative vector, Aedes albopictus, is spreading geographically into temperate climates, which could exacerbate the global dengue spread. Both mosquito species have been found in the United States.
“In 2001, the first dengue fever outbreak in Hawaii since World War II caused a lot of alarm, people got sick and there was great public concern,” Morens said. “We talk about ‘mild’ dengue, but people with classic dengue fever would probably dispute the word ‘mild.’”
The National Institute of Allergy and Infectious Diseases allocated $33.2 million in 2007 for nearly 60 dengue research projects, including vaccine, diagnostic and therapeutic development and clinical vaccine trials.
Dengue questions abound
Dengue fever is complex and multiple factors confound scientific answers, including risks and the lack of understanding about pathogenesis of dengue hemorrhagic fever and dengue shock syndrome.
Classic dengue fever symptoms are similar to those of influenza, but can confound diagnosis because symptoms are not predictive markers for disease severity, progression or hypotension. These variables make this virus complex and difficult to understand.
Risk for dengue hemorrhagic fever and dengue shock syndrome most often are associated with second dengue infections. However, dengue hemorrhagic fever in infants aged 5 to 8 months does not follow this pattern because the first, rather than the second infection, is more likely to progress beyond classic dengue. Risks for these more severe forms typically decrease with each subsequent infection, but this decrease also may depend on the virus serotype.
Other questions arise regarding vaccination development strategies because antibodies could decrease efficacy over time and vectors may develop resistance.
The National Institute of Allergy and Infectious Diseases’ dengue research program includes the development of animal models to gain understanding of the basic mechanisms of infection. Researchers also are working to understand how dengue viruses elicit antibodies, which is key to unlocking the mode through which viruses infect host cells. Research is underway on biological markers to gain insight into predictors of whether dengue fever will develop into dengue hemorrhagic fever or dengue shock syndrome.
Researchers also are trying to determine what causes capillaries to leak plasma in people infected with dengue hemorrhagic fever and dengue shock syndrome and investigating a possible genetic susceptibility to infection. Promising dengue vaccines have already reached phase 2 safety and efficacy testing including inactivated, live attenuated, chimeric, subunit and DNA vaccines, Morens and Fauci wrote.
For more information:
Morens DM, Fauci AS. Dengue and hemorrhagic fever: a potential threat to public health in the United States. JAMA. 2008;299:214-216.
Infectious Disease News
Saturday, February 16, 2008
An antidote for Staphylococcus aureus pneumonia?
Frank R. DeLeo and Michael Otto
F.R. DeLeo and M. Otto are at the Laboratory of Human Bacterial Pathogenesis, Rocky Mountain Laboratories, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Hamilton, MT 59840
CORRESPONDENCE F.R.D.: firstname.lastname@example.org
Methicillin-resistant Staphylococcus aureus (MRSA) is the leading cause of bacterial infections in the United States. Severe invasive MRSA infections, which include pneumonia, are difficult to treat because the bacteria are resistant to antibiotics. A new report now shows that immunization against -hemolysin (Hla), a cytolytic toxin secreted by most S. aureus strains, protects mice against lethal pneumonia. This finding represents the first successful vaccine strategy for the treatment of staphylococcal pneumonia.
S. aureus is a leading cause of bloodstream, skin, soft tissue, and lower respiratory tract infections worldwide (1). In developed countries such as the United States, resistance to β-lactam antibiotics in MRSA is a major problem in hospitals and other healthcare settings. In these settings, S. aureus infections are primarily caused by MRSA and typically occur in individuals with risk factors for disease, such as those who are immunocompromised or have had surgery. Notably, the incidence rate of all invasive MRSA infections, including those outside of hospitals, is high compared with other bacterial pathogens (31.3 per 100,000 individuals), and 20% of these infections result in death (2). Although bacteremia is the most prevalent condition during invasive disease caused by MRSA, pneumonia ranks second and occurs in 13.3% of all invasive infections (2).
In contrast to S. aureus infections acquired in healthcare settings, community-associated S. aureus infections, which in the United States are also caused primarily by MRSA, occur in otherwise healthy individuals. The majority of community-associated MRSA (CA-MRSA) infections are treatable infections of skin and soft tissue (3), but some infections lead to severe invasive disease (4). CA-MRSA was first reported in the late 1990s, when pneumonia was the third most prevalent syndrome, occurring in 13.5% of infected children (5). The most prevalent CA-MRSA isolate, known as USA300, accounts for up to 97% of all CA-MRSA infections (6).
Past efforts to generate an effective vaccine against S. aureus have thus far been unsuccessful. A new report by Wardenburg and Schneewind on page in this issue (7) shows that immunization with the S. aureus virulence factor Hla protects mice from an otherwise lethal S. aureus infection.
Virulence factors and immune evasionS. aureus encodes a remarkable repertoire of virulence factors. These molecules promote host colonization, facilitate evasion of the human innate immune system, and alter immune responses (for review see reference 8). For the purposes of this commentary, we will limit our discussion to a few S. aureus surface molecules, some of which have been used previously as vaccine targets.
Human neutrophils are a primary cellular defense against bacterial infections. Previous studies have shown that host opsonins, such as serum complement and antibody, play a major role in the phagocytosis of S. aureus by neutrophils (9–11). S. aureus makes several molecules, including protein A, serotype 5 or 8 capsular polysaccharide (CP5 or CP8), and clumping factor A (ClfA), which inhibit phagocytosis (12–14). But despite the bacteria's efforts to evade neutrophils, normal human serum contains a sufficient number of opsonins to promote their rapid uptake by these cells (15). The majority of clinical isolates, including USA300, encode ClfA and CP5 or CP8 (14). Because antibodies specific for CP5 or CP8 enhance phagocytosis, CP5 and CP8 have been evaluated extensively as vaccine antigens (16–18). In the end, however, S. aureus vaccines designed to enhance bacterial uptake have had limited success.
One possible reason for this outcome is the lack of correlation between uptake of the bacteria by neutrophils and their subsequent destruction. For instance, the most prominent CA-MRSA isolates survive relatively well inside neutrophils, probably in part because of their ability to resist the effects of neutrophil-derived reactive oxygen species and antimicrobial peptides (19, 20). The neutrophils, on the other hand, undergo rapid lysis after uptake of these strains (Fig. 1) (15, 21). The ability of S. aureus to survive after phagocytosis has lead some to suggest that neutrophils could be a vector for disseminating bacteria (22, 23). S. aureus can also persist inside macrophages for several days, ultimately causing the death of the cells in a process that depends on Hla (24).
Because uptake does not necessarily correlate with the killing of S. aureus, high titers of anticapsule antibodies, which facilitate uptake, may not protect against disease. This notion is not new, as it has long been known that virtually all humans have circulating antistaphylococcal antibody, and yet some still become infected (25). The idea that antibodies against the bacterial capsule may not provide protection was borne out in two unsuccessful phase III clinical trials designed to test the efficacy of active immunization against the S. aureus antigens CP5, CP8, and ClfA (26).
Hla, a pore-forming cytolytic toxin that assembles as a heptameric β-barrel structure in the plasma membrane of susceptible cells, is arguably the most widely studied S. aureus toxin (for review see reference 27). The toxin is known to cause destruction of a wide-range of host cells, including erythrocytes, epithelial cells, fibroblasts, and monocytes. Hla gained notoriety in 1928 when it was implicated in the deaths of 12 Australian children who had received a diphtheria toxoid vaccine that was later found to be contaminated with an Hla-producing S. aureus strain (27). Although anti-Hla antibody therapy was studied intensively, interest in this approach waned during the antibiotic era. S. aureus encodes numerous other extracellular cytolytic toxins, including -hemolysin, -hemolysin, Panton-Valentine leukocidin (PVL), leukocidin D/E, a leukocidin homologue (LukM/F'-PV), and the newly described phenol-soluble modulin-like peptides (28). The relative contribution of Hla to human disease as compared with these other virulence factors is not known, in part because susceptibility to Hla varies among different animal species.
A vaccine approach for treatment of S. aureus pneumonia. Until now, vaccination against Hla has not been tested in an S. aureus pneumonia model. In this issue, Wardenburg and Schneewind show that immunization against Hla prevents S. aureus pneumonia (7). The authors first show that the severity of lung disease in mice correlates with the levels of Hla produced by a particular S. aureus isolate (7). These findings are consistent with a recent study from the same group demonstrating that Hla is important for the pathogenesis of CA-MRSA pneumonia (29).
In the new study, mice were immunized with a nonpore-forming Hla variant, HlaH135L, and challenged intranasally 3 wk later, a protocol that typically induces lethal pneumonia (29). Immunization with HlaH135L protected 90–100% mice against all S. aureus strains tested (7). Vaccine-induced protection correlated directly with reduced inflammation and less severe destruction of lung tissue. Passive immunization with Hla antibody 24 h before intranasal challenge with S. aureus also protected animals against an otherwise lethal intranasal challenge with CA-MRSA or an antibiotic-sensitive S. aureus strain (7).
Antibodies against Hla also protected human lung epithelial cells from S. aureus–induced lysis (7). Although these results indicate that Hla contributes to lung tissue destruction, it is not yet clear whether the animals' death resulted from direct destruction of lung cells by the toxin, from an excessive inflammatory response, or from both. Passive transfer of Hla antibodies significantly reduced circulating levels of interleukin 1β, a cytokine known to accompany acute lung injury. Therefore, it is reasonable to conclude that the inflammatory response may contribute to Hla-mediated lung damage (Fig. 2).
A role for other toxins?
There has been considerable debate about whether another S. aureus toxin, PVL, is essential for CA-MRSA virulence. In their report, Wardenburg and Schneewind found that, unlike antibodies specific for Hla, antibodies specific for PVL did not protect mice against S. aureus pneumonia (7). This finding was consistent with an earlier study by this group, which also suggested that PVL is not required for CA-MRSA–induced pneumonia in mice (29).
This result appears to conflict with earlier data suggesting an essential role for PVL (30). But that study relied on laboratory strains of S. aureus that overexpressed PVL, thus clouding the physiological significance of these findings. Indeed, studies conducted using animal models of CA-MRSA disease have unanimously suggested that PVL is dispensable for bacterial virulence (21, 29). Recent work has, however, highlighted the potential importance of other virulence factors in CA-MRSA disease (28, 29).
Bacterial toxins as vaccine targetsThere are numerous examples of successful vaccination against bacterial toxins, including botulinum, diphtheria, and tetanus toxins. However, these toxins are known to be the primary causative agents of disease induced by their respective organisms. In contrast, S. aureus produces many toxins, and it has been generally accepted that no single S. aureus extracellular molecule can trigger disease on its own. This idea is called into question by the finding that Hla alone is required for S. aureus pneumonia (7, 29). The high level of infections caused by the S. aureus isolate USA300 and the abundance of Hla produced by this strain in vitro suggest that targeting Hla during invasive CA-MRSA infections may be a promising therapeutic approach.
AcknowledgmentsThis article was supported in part by the Intramural Research Program of the National Institute of Allergy and Infectious Diseases, National Institutes of Health.
Journal of Experimental Medicine
Vaccine protection against Staphylococcus aureus pneumonia
Juliane Bubeck Wardenburg1,2 and Olaf Schneewind1
1 Department of Microbiology and 2 Department of Pediatrics, University of Chicago, Chicago, IL 60637
CORRESPONDENCE Olaf Schneewind: email@example.com
Staphylococcus aureus pneumonia causes significant mortality in hospitalized or healthy individuals, and recent increases in morbidity are attributed to the rapid spread of methicillin-resistant S. aureus (MRSA) strains, which are often not susceptible to antibiotic therapy. -Hemolysin (Hla), a secreted pore-forming toxin, is an essential virulence factor of MRSA in a mouse model of S. aureus pneumonia. We show that the level of Hla expression by independent S. aureus strains directly correlates with their virulence. Active immunization with a mutant form of Hla (HlaH35L), which cannot form pores, generates antigen-specific immunoglobulin G responses and affords protection against staphylococcal pneumonia. Moreover, transfer of Hla-specific antibodies protects naive animals against S. aureus challenge and prevents the injury of human lung epithelial cells during infection. Thus, Hla vaccination or immunotherapy may prevent S. aureus pneumonia in humans.
The Journal of Experimental Medicine
Sunday, February 03, 2008
A New Method for Antimicrobial Susceptibility Testing of In Vitro-cultured Bacteria By Means of Resonance Light Scattering Technique.
Shi YJ, Chen J, Xu M.
School of Chemistry and Chemical Engineering, Nantong University, Nantong, Jiangsu 226000, P.R. China.
A new method for antimicrobial susceptibility testing of in vitro-cultured bacteria on an ordinary fluorescence spectrometer was developed. The viable bacteria reduced 3- (4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) to produce insoluble particles that displayed intense resonance scattering light (RSL). The assay showed a linear relationship between the number of viable bacteria and the intensity of resonance scattering light. Dead bacteria were unable to reduce MTT. Methicillin-resistant Staphylococcus aureus exposed to flavonoids from Marchantia convoluta showed a flavonoids concentration-dependent inhibition of the ability to reduce MTT. In the assay, less than 12 h were required to attain susceptibility results and fewer bacteria were utilized than in traditional methods.
The RLS technique could, in combination with the MTT assay, be a rapid and sensitive measuring method to determine the in vitro activity of new antimicrobials.
PMID: 18239427 [PubMed - in process]