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Testing of Streptococcus pneumoniae for resistance to penicillin
K J Marshall1, D M Musher, D Watson
1Medical Service, Veterans Affairs Medical Center, Houston, Texas.
Abstract:
The increasing prevalence of penicillin-resistant Streptococcus pneumoniae requires antibiotic susceptibility tests that can be done with greater ease and reliability. We measured the MIC of penicillin for pneumococci by the tube macrodilution method with Mueller-Hinton broth (MHB), Haemophilus Test Medium (HTM), Todd-Hewitt broth with 0.5% yeast extract (THY), and MHB with 3% lysed horse blood (LHB). Eight (19%) and 6 (14%) of 42 pneumococcal isolates failed to generate turbid growth in MHB and HTM, respectively, whereas all pneumococcal isolates did so in THY and LHB. For those strains that replicated to turbidity, the mean MICs of penicillin were lower in MHB and HTM than in THY and LHB, with differences being significant (P < 0.05) for comparisons with LHB. Four isolates appeared to be penicillin susceptible in HTM but were actually moderately resistant in THY and LHB, and two isolates appeared to be moderately resistant but were resistant. A similar failure to detect resistance was seen with MHB. S. pneumoniae ATCC 49619, a moderately penicillin-resistant strain that has been proposed for quality control testing, gave variable results in MHB or THM and appeared to be susceptible to penicillin in some assays, whereas the MICs for S. pneumoniae ATCC 49619 in THY or LHB fell within a twofold dilution range, with geometric means of 0.16 and 0.18 micrograms/ml, respectively. Pneumococcal isolates thus may appear falsely susceptible to penicillin when tested in MHB or HTM. LHB remains the standard medium; however, because THY is an easily prepared clear medium that can be used in automated systems and appears to yield results similar to those obtained with LHB, THY deserves consideration for routine use.
Insights
Todd-Hewitt broth with yeast extract (THY) offers a reliable alternative for testing penicillin susceptibility in Streptococcus pneumoniae. This clear medium supports consistent growth and accurate minimum inhibitory concentration (MIC) results, unlike Mueller-Hinton broth or Haemophilus Test Medium.
Area of Science:
- Clinical Microbiology
- Antimicrobial Resistance
- Diagnostic Bacteriology
Background:
- Increasing penicillin resistance in Streptococcus pneumoniae necessitates improved antibiotic susceptibility testing methods.
- Current methods using Mueller-Hinton broth (MHB) and Haemophilus Test Medium (HTM) present challenges with pneumococcal growth and accurate resistance detection.
- Accurate minimum inhibitory concentration (MIC) determination is crucial for guiding effective antibiotic therapy against pneumococcal infections.
Purpose of the Study:
- To evaluate the suitability of different culture media for reliable penicillin susceptibility testing of Streptococcus pneumoniae.
- To compare the performance of Todd-Hewitt broth with yeast extract (THY) and MHB with lysed horse blood (LHB) against MHB and HTM.
- To identify a culture medium that ensures consistent pneumococcal growth and accurate MIC values for penicillin.
Main Methods:
- Tube macrodilution method was employed to determine penicillin MICs for 42 Streptococcus pneumoniae isolates.
- Four different broths were tested: MHB, HTM, THY, and LHB.
- Bacterial growth turbidity and penicillin MICs were recorded for each medium.
Main Results:
- MHB and HTM showed poor or absent pneumococcal growth (19% and 14% failure rates, respectively).
- THY and LHB supported consistent turbidity for all isolates.
- Penicillin MICs were significantly lower in MHB and HTM compared to THY and LHB, leading to false susceptibility reports.
- The quality control strain S. pneumoniae ATCC 49619 yielded variable results in MHB/HTM but consistent MICs in THY/LHB.
Conclusions:
- Mueller-Hinton broth and Haemophilus Test Medium can lead to underestimation of penicillin resistance in Streptococcus pneumoniae.
- Todd-Hewitt broth with yeast extract (THY) provides reliable growth and accurate MIC results, comparable to the standard LHB.
- THY is a viable, easily prepared, clear medium suitable for automated systems and warrants consideration for routine susceptibility testing.