Quantitative antimicrobial susceptibility test for Streptococcus pneumoniae using inoculum supplemented with whole defibrinated sheep blood

Distribution of Multidrug-Resistant Invasive Serotypes of Streptococcus pneumoniae during the Period 2007-2021 in Madrid, Spain

After the systematic use of conjugate vaccines, the invasive pneumococcal disease (IPD) was included into the Madrid Notifiable Diseases Surveillance System through an Epidemiological Surveillance Network. Furthermore, Streptococcus pneumoniae was included in the Spanish Plan of Antibiotic Resistance. The aim of this study was to analyse the multidrug-resistant (MDR) phenotype distribution among invasive strains of Streptococcus pneumoniae isolated during 2007-2021 from usually sterile clinical samples in Madrid, Spain. A total number of 7133 invasive pneumococcal isolates were studied during the period from February 2007 to December 2021. Serotyping was characterised using the Pneumotest-Latex and by the Quellung reaction. Antibiotic susceptibility testing to penicillin (PEN), erythromycin (ERY), and levofloxacin (LVX) was performed using the E-test according to the EUCAST guidelines and breakpoints. Combination of non-susceptibility to PEN at standard dosing regimen (PNSSDR), resistance to ERY (ERYR) and to LVX (LVXR) was considered to be multidrug-resistant at standard dosing regimen of penicillin (MRPSDR), whereas the combination of resistance to PEN (PENR), ERYR, and LVXR was considered multidrug-resistant (MDR). The number of MDRPSDR and or MDR strains in the entire population (n = 7133) during the complete period (2007-2021) were 51 (0.7%) and 6 (0.1%), respectively. All MDRPSDR and/or MDR strains belonged to nine serotypes: 19A (n = 13), 15A (n = 12), 9V (n = 12), 14 (n = 7), 24F (n = 3), 15F (n = 1), 19F (n = 1), 6B (n = 1) and 6C (n = 1). Only two serotypes (9V and 19A) were found among MDR strains, and most of them (5/6) belonged to serotype 9V. Only 12.4% of the strains typified as serotype 9V were MDRPSDR and only 5.2% as MDR. The levels of pneumococcal MDRPSDR and/or MDR in this study were low and all six MDR strains were isolated between 2014 and 2018. These results reinforce the importance of monitoring the evolution of non-susceptible serotypes including those with MDR in the coming years, especially after the introduction of new conjugate vaccines of a broader spectrum.

Determination of penicillin MICs for Streptococcus pneumoniae by using a two- or three-disk diffusion procedure

The potential for the use of the disk diffusion method to accurately predict penicillin MICs for Streptococcus pneumoniae was investigated with penicillin (6 microg), methicillin (5 microg), and oxacillin (1 microg) disks. A total of 183 S. pneumoniae isolates were tested by three MIC procedures (agar dilution, microdilution, and E-test). Regression analyses of the geometric mean of the three MIC results against (i) the sum of the zone diameters for methicillin, penicillin, and oxacillin disks; (ii) the sum of the zone diameters for methicillin and penicillin disks; and (iii) each of the three individual zone diameters were performed. Calculated MICs were determined from each of these regression analyses and compared to the mean reference MICs. A high level of correlation was obtained with both the two- and the three-disk procedures (r = 0.97), with essential agreement rates (+/-1 doubling dilution) between MICs calculated by the three-disk procedure and the two-disk procedure and the mean reference MICs of 98.4 and 98.9%, respectively. No major or very major errors were obtained with the two- or three-disk procedures. The accuracy of the disks used individually was lower (r = 0.84 to 0.93). However, oxacillin and methicillin disk testing remain excellent for screening strains, with all penicillin-susceptible strains having zones of >21 and >22 mm, respectively. The combination disk procedure, which involves the use of three disks (methicillin, oxacillin, and penicillin) or two disks (methicillin and penicillin) for testing S. pneumoniae, can provide accurate penicillin MICs and qualitative category results that are comparable to results obtained by the E-test, agar, and microdilution MIC methods.

Comparison of the serum-supplemented Todd-Hewitt and the new Haemophilus test media for broth microdilution susceptibility testing of Streptococcus pneumoniae

Horse serum-supplemented Todd-Hewitt broth (STH) in use at Hôpital Ste-Justine for the last 12 years was compared to the recently proposed Haemophilus test medium (HTM), for broth microdilution susceptibility testing of Streptococcus pneumoniae. One hundred and twenty S. pneumoniae isolates from pediatric clinical specimens were used in this study. In general, the minimum inhibitory concentrations (MICs) in STH for 15 antimicrobial agents were quite comparable to those determined in HTM but tended to be higher. Drugs which generated MICs within +/- 1 log2 concentration differences in both media included penicillin, ampicillin, oxacillin, cefuroxime, cefotaxime, cefixime, clindamycin, chloramphenicol, trimethoprim-sulfamethoxazole, rifampin, ciprofloxacin and vancomycin. Cefaclor and tetracycline MICs tended to be > or = 2 log2 dilutions higher with STH for most of the isolates tested, while erythromycin MICs were often 2 log2 dilutions lower with STH than with HTM. Despite some differences in MICs noted above, few very major (0.4%), major (0.2%) and minor interpretive category errors (4.4%) were observed. The visual reading of the MICs for most of the 120 clinical isolates tested was generally easier in STH which was superior in supporting best the bacterial growth as detected by spectrophotometry. The risk of false susceptibility is thus decreased by using STH rather than HTM; furthermore, STH is free of the technical problems of the lysed horse blood Mueller-Hinton (LHB-MH) recommended by the NCCLS.

Treatment of experimental endocarditis due to penicillin-resistant Streptococcus pneumoniae

Using two strains of pneumococci for which MICs of penicillin were 1 and 4 micrograms/ml, those of cefotaxime were 0.01 and 0.5 micrograms/ml, and those of teicoplanin were 0.01 and 0.1 micrograms/ml, we studied the efficacy of different dosages of penicillin, cefotaxime, and teicoplanin in the treatment of experimental pneumococcal endocarditis in rabbits. Animals treated with dosages of penicillin G procaine needed to achieve levels in serum near the MIC for pneumococci showed a significant reduction in log10 CFU per gram of vegetation, as compared with the control (P < 0.001), although only 20% of the animals showed sterile vegetations. When levels of penicillin in serum were in the range of three- to fourfold the MIC, a greater reduction in log10 CFU per gram of vegetation was seen, and 88% of the animals showed sterile vegetations. Only the regimen of penicillin that provided concentrations in serum above the MIC throughout the interval between two doses provided constant sterilization of the cardiac vegetations. Dosages of cefotaxime and teicoplanin selected to achieve concentrations in serum equivalent to that obtained in humans during treatment resulted in levels of antimicrobial agents in serum hundreds or thousands of times higher than the MICs for the infecting strains. In terms of antimicrobial efficacy, cefotaxime and teicoplanin were equivalent to regimens with high dosages of penicillin.

Comparison of susceptibility test methods to detect penicillin-resistant Streptococcus pneumoniae

The detection of penicillin-resistant Streptococcus pneumoniae was assessed by six different methods: agar dilution, oxacillin screen by disk diffusion, E-test, and three overnight microdilution test methods that included commercial panels from MicroScan and Micro Media and in-house-made conventional panels using a commercial Haemophilus test medium (HTM) broth. Of the 52 pneumococcal isolates tested, 12 were resistant, 16 were relatively resistant, and 24 were susceptible to penicillin as defined by the reference agar dilution method. The oxacillin screen detected as resistant all 28 resistant and relatively resistant strains. The percentage of penicillin-resistant isolates detected by each minimum inhibitory concentration (MIC) test method was as follows: E-test (100%), Micro Media (75%), MicroScan (0%), and HTM (0%). With the relatively resistant isolates, the detection percentage was as follows: E-test (88%), Micro Media (94%), MicroScan (69%), and HTM (69%). In conclusion, the E-test and Micro Media MIC tests are acceptable confirmatory tests for detecting penicillin resistance among S. pneumoniae isolates.

Bacteremic pneumonia caused by penicillin-resistant pneumococci: Case report and review with a Canadian perspective

A Canadian adult with bacteremic pneumonia caused by a relatively penicillin-resistant (minimal inhibitory concentration 0.25 μg/mL) Streptococcus pneumoniae is reported, and the published literature regarding penicillin-resistant pneumococci in Canada reviewed. Although penicillin resistance has been reported infrequently to date, this case emphasizes the need for routine antimicrobial sensitivity testing of all pneumococci isolated from normally sterile sites, and for ongoing systematic surveillance for penicillin and other antibiotic resistance in Canada.

Pneumococcal resistance to antibiotics

The geographic distribution of pneumococci resistant to one or more of the antibiotics penicillin, erythromycin, trimethoprim-sulfamethoxazole, and tetracycline appears to be expanding, and there exist foci of resistance to chloramphenicol and rifampin. Multiply resistant pneumococci are being encountered more commonly and are more often community acquired. Factors associated with infection caused by resistant pneumococci include young age, duration of hospitalization, infection with a pneumococcus of serogroup 6, 19, or 23 or serotype 14, and exposure to antibiotics to which the strain is resistant. At present, the most useful drugs for the management of resistant pneumococcal infections are cefotaxime, ceftriaxone, vancomycin, and rifampin. If the strains are susceptible, chloramphenicol may be useful as an alternative, less expensive agent. Appropriate interventions for the control of resistant pneumococcal outbreaks include investigation of the prevalence of resistant strains, isolation of patients, possible treatment of carriers, and reduction of usage of antibiotics to which the strain is resistant. The molecular mechanisms of penicillin resistance are related to the structure and function of penicillin-binding proteins, and the mechanisms of resistance to other agents involved in multiple resistance are being elucidated. Recognition is increasing of the standard screening procedure for penicillin resistance, using a 1-microgram oxacillin disk.

Use of Haemophilus test medium for broth microdilution antimicrobial susceptibility testing of Streptococcus pneumoniae

A recently described medium (Haemophilus test medium [HTM]) for antimicrobial susceptibility testing of Haemophilus influenzae was evaluated in this study for broth microdilution testing of Streptococcus pneumoniae. A total of 137 clinical isolates was tested against 11 antimicrobial agents, using Mueller-Hinton broth supplemented with 3% lysed horse blood in parallel with HTM. Inocula of 5 X 10(5) CFU/ml and incubation for 20 to 24 h were used with both media. All isolates of S. pneumoniae produced acceptable growth in both media, and MICs determined in HTM agreed closely with those determined in lysed horse blood. Drugs which provided a MIC within 1 log2 concentration difference in both media included penicillin (100%), ampicillin (98.0%), amoxicillin-clavulanate (100%), ampicillin-sulbactam (100%), cephalexin (98.9%), cefaclor (96.8%), cefuroxime (99.0%), chloramphenicol (96.2%), tetracycline (96.2%), and erythromycin (100%). HTM MICs with trimethoprim-sulfamethoxazole were 1 to 2 log2 concentration increments higher in 92.0% of isolates than MICs determined in lysed horse blood. Based on the results of this study, HTM appears to represent a promising alternative medium for broth microdilution susceptibility testing of S. pneumoniae.