Pathogenesis of pneumococcal infection

The pathogenesis of pneumococcal infection is a complex interplay between pneumococcal virulence determinants and the host immune response. Molecular studies have considerably advanced our knowledge and understanding of the precise structures and functions of the different determinants and their pathogenic roles. This review describes the mechanisms by which pneumococci attach, invade, evade lung defences and cause severe disease. Better understanding of the critical steps in this complex process will enable more effective clinical intervention to be developed to reduce the mortality exacted by this versatile pathogen.

Comparison of phenotypic and genotypic methods for pyrazinamide susceptibility testing with Mycobacterium tuberculosis

Mycobacterium tuberculosis converts pyrazinamide to its active form by using the enzyme pyrazinamidase. This enzyme is coded for on the pncA gene, and mutations in the pncA gene result in absence of active enzyme, conferring resistance to the drug pyrazinamide. We investigated 27 strains of Mycobacterium tuberculosis suspected of being multidrug resistant. Each isolate was tested for susceptibility to pyrazinamide by the BACTEC 460TB method, and 19 were pyrazinamide resistant. The presence of active pyrazinamidase enzyme was sought by using the Wayne assay, which was positive in all of the sensitive isolates and four of the resistant isolates. The pncA gene was amplified by PCR, and mutations were sought by single-strand conformation polymorphism (SSCP) analysis. We identified four isolates which were phenotypically resistant to pyrazinamide, but which had active pyrazinamide enzyme on the Wayne assay and normal pncA gene SSCP. MICs measured by BACTEC 460TB and susceptibility testing at a lower pH of 5.5 confirmed genuine resistance. The pncA gene was sequenced in these four isolates and found not to have any mutations. This implies that an alternative mechanism of resistance exists in these strains. We conclude that genotypic assessment of pyrazinamide resistance is unreliable, because it depends on the identification of a single resistance mechanism. Phenotypic methods such as the BACTEC 460TB technique remain the best methods for pyrazinamide susceptibility testing.

Comparison of fitness of two isolates of Mycobacterium tuberculosis, one of which had developed multi-drug resistance during the course of treatment

OBJECTIVES

We report the cases of two patients, brother and sister, both with pulmonary tuberculosis. Both patients complied poorly with treatment. One developed multi-drug resistant disease, whilst the other did not. We aimed to show that the two infecting strains were the same, and then to compare the fitness of the resistant strain to that of the sensitive strain.

METHODS

The isolates were typed by RFLP. The fitness of the multi-drug resistant tuberculosis strain was determined by calculating the ratio of generation produced by the drug-resistant and a drug-susceptible strain in a mixed culture. The number of bacteria present in this broth culture was estimated using the Miles and Misra technique. The number of drug-resistant bacteria present was determined by inoculating aliquots of broth onto Middlebrook 7H10 agar with 5mg/l rifampicin.

RESULTS

The infecting strain of Mycobacterium tuberculosis was shown to be the same on RFLP typing in both cases. It was found that the multi-drug resistant organism had decreased fitness compared to the sensitive organism.

CONCLUSION

The decreased relative fitness of the resistant strain implies a physiologic cal cost for the development of drug resistance.

False molecular clusters due to nonrandom association of IS6110 with Mycobacterium tuberculosis

We sought to determine whether nonrandom association of IS6110 with Mycobacterium tuberculosis could result in false-positive clustering in unselected collections of isolates. We typed 196 strains of M. tuberculosis from an unselected community-based study in northern Tanzania by IS6110 and polymorphic GC-rich repetitive-sequence (PGRS) methodologies. The strains were analyzed by Gelcompar computer software. Analysis of 13 out of 25 groups showed that isolates with identical IS6110 and PGRS patterns were likely to be the same strain. Some IS6110 groups containing strains with identical PGRS patterns had similar IS6110 patterns that differed only by movement of the element. Isolates assigned to a single group (i.e., group 11) on the basis of sharing an identical IS6110 fingerprint pattern did not share identical PGRS fingerprint patterns. Six out of the nine bands in these isolates were in hot-spot locations, as previously defined. This indicates that nonrandom association may result in false-positive clustering in unselected community-based studies. Only strains with identical PGRS and IS6110 patterns are likely to be recently transmitted.

Early predictors of mortality in pneumococcal bacteraemia

OBJECTIVES

Pneumococcal bacteraemia carries a mortality of about 20%. Approximately 50% of deaths from pneumococcal bacteraemia occur within the first 48 h of admission. In order to influence outcome, critically ill patients should be identified at the time of presentation. This study enables the clinician to rapidly make an evidence-based assessment of a patient's prognosis, allowing the identification of patients who should be placed in a high-risk category at an early stage, when appropriate management is most likely to be effective.

METHODS

Data were collected from the medical record of history, physical examination, radiological examination and laboratory investigations done on initial presentation using a standardized proforma. The data were first examined by Pearson's Chi-squared test, with Yates' correction if needed. Variables found to be significantly associated with case fatality ( P < 0.05) by these methods were examined by stepwise logistic regression analysis in order to identify those factors which were independent predictors of case fatality.

RESULTS

The overall case fatality was 21%. Older age, apyrexia, tachypnoea, bilateral consolidation, hypoalbuminaemia, elevated aminotransferases, renal impairment, acidosis and leucopaenia were significantly associated with higher case fatality. Older age, acidosis and elevated serum alanine aminotransferase (ALT) were independent predictors of case fatality. Fifty-five percent of isolates belonged to serotypes 4, 6B, 9V, 14, 19F and 23F, to which good antibody levels have been documented in both young and elderly patients post-vaccination. Serotype 14 was most common, and was significantly associated with higher case fatality. Colder weather was associated with a higher incidence of both infection and case fatality. The case fatality amongst patients receiving ITU management was 44%. Less than 50% of patients who died received ITU management.

CONCLUSIONS

Despite the increased availability of new antibiotics and vaccines, the mortality of patients with pneumococcal bacteraemia remains unchanged. The parameters above allow early identification of patients with a higher case fatality; these patients may benefit from being placed in a "high-risk" category early on in their management. Vaccination of the elderly may reduce the incidence and/or mortality from pneumococcal bacteraemia. Further studies are required to understand the reasons for referral for intensive therapy in acute pneumococcal bacteraemia and whether ITU management affects outcome.

Length of time to laboratory diagnosis of Mycobacterium tuberculosis infection: comparison of in-house methods with reference laboratory results

OBJECTIVES

To audit the time taken to obtain laboratory confirmation of infection with Mycobacterium tuberculosis using in-house methods of polymerase chain reaction (PCR) and culture and referral to a reference laboratory.

METHODS

Retrospective collection of data from laboratory records covering a period of 1 year.

RESULTS

Median time to microbiological diagnosis of a new infection using the in-house services in addition to the reference laboratory was 22.0 days. Using reference laboratory results alone, median time to diagnosis would have been 61.5 days.

CONCLUSIONS

Development of on-site laboratory facilities to identify Mycobacterium tuberculosis can reduce the time to its identification by almost two-thirds.

Activity of moxifloxacin against mycobacteria

Moxifloxacin is an 8-methoxyquinolone compound with activity against a wide range of bacteria. We tested its activity in comparison with four other quinolones and isoniazid against clinical isolates of mycobacteria. It proved to be the most active of the quinolones tested against Mycobacterium tuberculosis (MIC90 0.25 mg/L), Mycobacterium avium-intracellulare (MIC90 1.0 mg/L), Mycobacterium kansasii (MIC90 0.06 mg/L) and Mycobacterium fortuitum (MIC90 1 mg/L). These data indicate that moxifloxacin merits further study as an antimycobacterial agent.

The role of the molecular laboratory in the investigation of Streptococcus pneumoniae infections

The importance of infections with Streptococcus pneumoniae is being increasingly recognized. This has been emphasised by the emergence and international dissemination of penicillin-resistant strains. These factors challenge modern microbiologists to use molecular tools to speed the process of the diagnosis of S pneumoniae at each of the stages: rapid diagnosis, culture identification, and typing. The polymerase chain reaction (PCR) has been used to amplify a number of different targets for primary diagnosis of pneumonia, meningitis, otitis media, and bacteraemia. This may provide a rapid diagnosis in patients who have been partially treated with antibiotics and increase the diagnostic yield in clinical trials of antimicrobial agents and new vaccines. A new development is the use of PCR to rapidly determine penicillin susceptibility. Several different experimental protocols are reviewed. In the past serotyping using the capsule swelling, or Quellung reaction was the only typing method available. Recent insight into epidemiology and population genetics of S pneumoniae has shown that this technique is inadequate to study the transmission of this organism. New molecular typing techniques have been developed including Field version gel electrophoresis, ribotyping, restriction fragment end labeling, BOX PCR, and random amplification of polymorphic DNA. As these methods become available to clinicians, they will gain more understanding of the way in which this organism interacts with human populations.

Physiological cost of rifampin resistance induced in vitro in Mycobacterium tuberculosis

Drug-resistant Mycobacterium tuberculosis is a major threat to public health. In clinical practice, a limited number of resistance mutations in a short sequence of the beta subunit of RNA polymerase (encoded by rpoB) have been described. Spontaneous resistance to rifampin was induced in vitro in M. tuberculosis H37Rv (ATCC 9360). Only three resistance patterns could be detected by PCR-single-strand conformation polymorphism analysis. Sequence analysis revealed that Ser(531)-->Leu arose most frequently, followed by His(526)-->Arg and then either His(526)-->Tyr or His(526)-->Asp. The relative Darwinian fitness of all but one of the mutant genotypes was less than that of the susceptible parent and, for these mutations, there was a significant correlation between fitness and clinical isolation rate (regression analysis P = 0.026). The fitness deficit in some mutants was small, suggesting that there is little likelihood of a spontaneous reversion to susceptibility.

Detection of penicillin susceptibility in Streptococcus pneumoniae by pbp2b PCR-restriction fragment length polymorphism analysis

A PCR-restriction fragment length polymorphism strategy directed against the pbp2b gene was evaluated for identification of penicillin susceptibility. A total of 106 United Kingdom (U.K.), 30 Danish, and 11 Papua New Guinean strains were tested. Of the U.K. strains, all the susceptible and all but one of the resistant isolates were correctly assigned. By using conventional definitions of "not resistant" and "not susceptible," the sensitivities were 97. 5 and 94.4%, the specificities were 100 and 98.9%, the positive predictive values were 100 and 94.4%, and the negative predictive values were 93.1 and 98.9%, respectively. This technique may allow susceptible (MIC, <0.1 mg/liter) and resistant (MIC, >1 mg/liter) isolates to be distinguished in a single PCR.

RP59500 (Quinupristin/dalfopristin): three case reports of its use in infection due to Enterococcus faecium

We describe three cases of Enterococcus faecium sepsis arising in immunocompromised patients, severely ill with other conditions, who were treated with the new injectable streptogramin RP59500. There are still few reports of clinical experience with this drug. All had bacteriological resolution, with one patient recovering fully. Although two of the three patients died, this was due to underlying disease in one case and a gram-negative superinfection in another. Quinupristin/dalfopristin therapy was not associated with significant adverse effects in any of the patients.

Nonrandom association of IS6110 and Mycobacterium tuberculosis: implications for molecular epidemiological studies

IS6110 restriction fragment length polymorphism typing is now established as the primary typing method for Mycobacterium tuberculosis. It has been assumed that the position of bands is random. Thus, the discrimination of the technique increases in proportion to the copy number. Two collections of M. tuberculosis were investigated to test this hypothesis. We identified 33 positions in isolates from a Tanzanian collection and 25 positions in isolates from a London, United Kingdom, collection where bands were significantly more likely to be present than would be expected by chance. These data suggest that band position is not random, and this possibility may have an impact on the interpretation of molecular epidemiological studies of M. tuberculosis.

Fluoroquinolones: a new treatment for tuberculosis?

The fluoroquinolones have secured an important place in the management of bacterial infection, they are well absorbed orally, are found in respiratory secretions in higher concentrations than in serum and are concentrated inside macrophages. The agents are well tolerated and have an excellent safety record in long-term therapy. No new antituberculosis agents have been developed since the introduction of rifampicin into clinical use, so fluoroquinolones have been investigated for potential efficacy in tuberculosis. In vitro studies have shown that they are active against Mycobacterium tuberculosis at achievable concentrations. Treatment studies in mice have demonstrated efficacy. Few clinical studies have been performed in humans, but ciprofloxacin has demonstrated significant early bactericidal activity. Regimens including a fluoroquinolone have been comparable to other standard regimens, although the outcome in human immunodeficiency virus (HIV) seropositive patients was significantly poorer. There is still insufficient clinical data to use fluoroquinolones in first-line treatment of tuberculosis, but they may find a role in the management of multidrug-resistant infections or in patients with adverse reactions to other agents.

New polymerase chain reaction-based diagnostic techniques for bacterial respiratory infection

Polymerase chain reaction protocols are now available for the diagnosis of all of the major bacterial respiratory tract pathogens. Molecular techniques are also being used to determine the susceptibility of Streptococcus pneumoniae and Mycobacterium tuberculosis. In addition, polymerase chain reaction-based diagnosis will find a place in the investigation of the epidemiology of some pathogens such as Bordetella pertussis and Chlamydia pneumoniae. The added sensitivity will allow the detection of mild or asymptomatic cases or carriers who may play a role in sustaining the pathogen in the community.

An outbreak of penicillin resistant Streptococcus pneumoniae investigated by a polymerase chain reaction based genotyping method

AIMS

To characterise the genotypes of penicillin resistant Streptococcus pneumoniae infecting patients in a care of the elderly ward and to study its transmission in a hospital environment.

METHODS

Isolates of S pneumoniae were cultured from specimens obtained from patients who had been admitted to a care of the elderly ward where an outbreak had occurred. Penicillin resistant S pneumoniae were also obtained from a series of surveillance throat swabs taken from patients in the same ward. In addition, all penicillin resistant S pneumoniae isolated from specimens submitted for culture at the time of the outbreak were included. Four sensitive strains isolated from a routine microbiology laboratory were included as controls. A simple polymerase chain reaction (PCR) based genotyping method for the penicillin binding protein (PBP) genes 1a, 2x, and 2b was used to characterise the genotypes.

RESULTS

Nine patients were infected with serotype 9 S pneumoniae. Four of these patients died; two deaths were directly attributable to the infection. Tested against a battery of haemolytic streptococci and other organisms found in the respiratory tract, only two false positive reactions for PBP 2x were found among S mitis. The method demonstrated that the outbreak strain had altered PBP 1a, 2b, and 2x genes, a pattern clearly distinguishable from other penicillin resistant strains isolated at the same time.

CONCLUSIONS

This method is simple to perform and would enable many laboratories to characterise the genotype of penicillin resistant S pneumoniae and investigate transmission in their hospitals.

Allelic variation in Streptococcus pneumoniae autolysin (N-acetyl muramoyl-L-alanine amidase)

The lytA gene encoding the autolysin of Streptococcus pneumoniae may be a virulence determinant. Single-strand conformational polymorphism analysis demonstrated heterogenicity throughout the gene in clinical isolates and strains from the clonal serotypes 7 and 14. Sequence analysis of part of the choline-binding domain showed that in two isolates four amino acid substitutions occurred.

IS6110 homologs are present in multiple copies in mycobacteria other than tuberculosis-causing mycobacteria

We have previously demonstrated homology between a 181-bp fragment of IS6110 and DNA from mycobacteria other than tuberculosis-causing mycobacteria (MOTT). Genomic DNA from 14 strains of MOTT was digested with PvuII and was hybridized with a probe derived from the 181-bp fragment and the INS1/INS2 international standard probe at high stringency. Multiple banding patterns were obtained from isolates of M. avium-M. intracellulare, M. fortuitum, M. kansasii, and M. malmoense. Differences in the banding patterns between and within species were obtained. This suggests that mycobacteria possess a family of IS3-like elements. The species of isolates suspected of being M. tuberculosis must be carefully determined before IS6110 restriction fragment length polymorphism analysis, and caution must be used in designing and evaluating diagnostic PCR tests based on this element.