Molecular DNA analysis for differentiation of persistence or relapse from recurrence in treatment failure of Streptococcus pyogenes pharyngitis

Phenotypic Resistance to Antibiotics

The development of antibiotic resistance is usually associated with genetic changes, either to the acquisition of resistance genes, or to mutations in elements relevant for the activity of the antibiotic. However, in some situations resistance can be achieved without any genetic alteration; this is called phenotypic resistance. Non-inherited resistance is associated to specific processes such as growth in biofilms, a stationary growth phase or persistence. These situations might occur during infection but they are not usually considered in classical susceptibility tests at the clinical microbiology laboratories. Recent work has also shown that the susceptibility to antibiotics is highly dependent on the bacterial metabolism and that global metabolic regulators can modulate this phenotype. This modulation includes situations in which bacteria can be more resistant or more susceptible to antibiotics. Understanding these processes will thus help in establishing novel therapeutic approaches based on the actual susceptibility shown by bacteria during infection, which might differ from that determined in the laboratory. In this review, we discuss different examples of phenotypic resistance and the mechanisms that regulate the crosstalk between bacterial metabolism and the susceptibility to antibiotics. Finally, information on strategies currently under development for diminishing the phenotypic resistance to antibiotics of bacterial pathogens is presented.

Metabolic regulation of antibiotic resistance

It is generally assumed that antibiotics and resistance determinants are the task forces of a biological warfare in which each resistance determinant counteracts the activity of a specific antibiotic. According to this view, antibiotic resistance might be considered as a specific response to an injury, not necessarily linked to bacterial metabolism, except for the burden that the acquisition of resistance might impose on the bacteria (fitness costs). Nevertheless, it is known that changes in bacterial metabolism, such as those associated with dormancy or biofilm formation, modulate bacterial susceptibility to antibiotics (phenotypic resistance), indicating that there exists a linkage between bacterial metabolism and antibiotic resistance. The analyses of the intrinsic resistomes of bacterial pathogens also demonstrate that the building up of intrinsic resistance requires the concerted action of many elements, several of which play a relevant role in the bacterial metabolism. In this article, we will review the current knowledge on the linkage between bacterial metabolism and antibiotic resistance and will discuss the role of global metabolic regulators such as Crc in bacterial susceptibility to antibiotics. Given that growing into the human host requires a metabolic adaptation, we will discuss whether this adaptation might trigger resistance even in the absence of selective pressure by antibiotics.

Non-inherited antibiotic resistance

In addition to their impressive, well-publicized and well-researched propensity to evolve and acquire genetically determined mechanisms for resistance to antibiotics, bacteria that are inherently susceptible to these drugs can also be phenotypically refractory to their action. This phenomenon of 'non-inherited resistance' to antibiotics has been known since the beginning of the antibiotic era but, relative to inherited resistance, it has been given little attention. Here, we review the in vitro and in vivo evidence for the different forms of non-inherited resistance and the mechanisms responsible. With the aid of a simple mathematical model and computer simulations, we show how non-inherited resistance could extend the duration of antibiotic treatment, cause treatment failure and promote the generation and ascent of inherited resistance in treated patients.

Emergence of group A streptococcus strains with different mechanisms of macrolide resistance

The mechanisms of resistance to macrolides in seven group A streptococcal (Streptococcus pyogenes) isolates that were the cause of pharyngitis in children who were unsuccessfully treated with azithromycin (10 mg/kg of body weight/day for 3 days) were evaluated. All posttreatment strains were found to be genetically related to the pretreatment isolates by random amplified polymorphism DNA analysis and pulsed-field gel electrophoresis. Two isolates had acquired either a mef(A) or an erm(B) gene, responsible for macrolide efflux and ribosomal modification, respectively. Three isolates displayed mutations in the gene encoding the L4 ribosomal protein that is part of the exit tunnel within the 50S subunit of the bacterial ribosome. In the two remaining posttreatment strains, the mechanisms of macrolide resistance could not be elucidated.

Comparison of two dosages of azithromycin for three days versus penicillin V for ten days in acute group A streptococcal tonsillopharyngitis

BACKGROUND

Three-day, 10 mg/kg/day azithromycin (AZM) studies in pediatric acute group A streptococcal tonsillopharyngitis have shown contradictory bacteriologic results. This study investigates the efficacy and tolerability of two dosages of 3-day azithromycin (20 mg/kg/day and 10 mg/kg/day) compared with 10-day penicillin V.

METHODS

This was a prospective, comparative, randomized, multicenter trial. Children were scheduled to return for visits at 14 days (main end point) and 1 month after the onset of treatment for clinical and bacteriologic assessment. Molecular tools were used to compare pre- and posttreatment group A beta-hemolytic Streptococcus (GABHS) isolates.

RESULTS

Between November, 1997, and July, 1998, 501 patients (169 AZM 10 mg, 165 AZM 20 mg, 167 penicillin V) between 2 and 12 years old were enrolled; 500 were assessable for safety, 469 for intent to treat analysis and 420 for efficacy in the per protocol analysis. Before treatment 25 (7.9%) of 315 GABHS stains isolated from patients receiving AZM were resistant to this compound. On Day 14 pretreatment GABHS were eradicated from 78 (57.8%) of the 135 children receiving the AZM 10 mg regimen, 131 (94.2%) of the 139 receiving AZM 20 mg and 123 (84.2%) of the 146 taking penicillin. One month after the outset of treatment, bacteriologic relapses were observed in 40.5% (n = 30) of the children receiving AZM 10 mg, 14.8% (n = 18) of children taking AZM 20 mg and 13.2% (n = 15) of those treated with penicillin V. AZM 20 mg/kg/day was statistically superior to AZM 10 mg/kg/day microbiologically on Day 14 (P = 0.0001) and Day 30 (P = 0.0001) and clinically on Day 14 (P = 0.0035). AZM 20 mg/kg/day was statistically equivalent both microbiologically and clinically to standard therapy with penicillin V at all endpoints. The incidence of treatment-related adverse events was similar in the two azithromycin groups [AZM 10 mg, 31 of 169 (18.3%); AZM 20 mg, 37 of 164 (23%)] but significantly higher than those observed in the penicillin V group [5 of 166 (3%); P < 0.0001]. Most treatment-related adverse events were gastrointestinal and of mild-to-moderate severity. Fourteen patients withdrew from the trial because of adverse events (1 in the penicillin V group, 7 in the AZM 10 mg group and 6 in the AZM 20 mg group).

CONCLUSION

This is the first study to demonstrate a daily dose-dependent difference in microbiologic efficacy of a regimen; 3-day AZM 20 mg/kg/day is a more effective regimen than 3-day AZM 10 mg/kg/day for pediatric GABHS tonsillopharyngitis.

Resistance to macrolides in Streptococcus pyogenes in France in pediatric patients

A total of 1,500 recent throat isolates of Streptococcus pyogenes collected between 1996 and 1999 from children throughout France were tested for their susceptibility to erythromycin, azithromycin, josamycin, clindamycin, and streptogramin B. The erythromycin-resistant isolates were further studied for their genetic mechanism of resistance, by means of PCR. The clonality of these strains was also investigated by means of serotyping and ribotyping. In all, 6.2% of the strains were erythromycin resistant, and 3.4 and 2.8% expressed the constitutive MLS(B) and M resistance phenotypes and harbored the ermB and mefA genes, respectively; ermTR was recovered from one isolate which also harbored the ermB gene. Ten serotypes and 8 ribotypes were identified, but we identified 17 strains by combining serotyping with ribotyping. Among the eight ribotypes, the mefA gene was recovered from six clusters, one being predominant, while the ermB gene was recovered from four clusters, of which two were predominant.

[Streptococcus pyogenes: in vitro susceptibility and role of beta-lactamase producing bacteria in the persistence of streptococcal pharyngotonsillitis]

OBJECTIVE

To assess the frequency of association between Streptococcus pyogenes and beta-lactamase-producing-bacteria in the pharyngotonsillitis and the evaluate the in vitro susceptibility.

DESIGN

Prospective, descriptive, transverse study.

SETTING

The present study was carried out in the Health Center Dr. José Castro Villagrana, in Tlalpan, México, D.F., from Juanary, 1996 to February 1999.

PARTICIPANTS

In three hundred and ninety four patients with pharyngotonsillitis diagnosis we isolated the same number of Streptococcus pyogenes, and possible beta-lactamase-producing-bacteria.

RESULTS

In 180 patients (45.7%) we isolated at least one possible beta-lactamase-producing-bacteria. Of these, in 138 patients (35%) were confirmed the enzyme presence. In total, we isolated 218 possible beta-lactamase-producing bacteria, and 152 (69.7%) were beta-lactamase positive. We found no significant change in the in vitro susceptibility of group A Streptococcus to penicillin, but erythromycin resistance is relatively common, approximately 10% in this study.

CONCLUSIONS

Streptococcus pyogenes was uniformly susceptible to all penicillins and cephalosporins in vitro. Erythromycin treatment should not be promoted as first-line therapy because the consequent increase of bacterial resistance could create difficulty in treating penicillin-allergic patients. Because of the poor activity of trimetoprimsulfametoxazol, this drug no longer can be considered the drug of choice for the management of group A Streptococcal infections.