Mark S. Miller, Microbiology

During the Winter Semester of 1994, I was able to work with three members of the genus Streptococcus: S. pneumoniae, S. agalactiae, and S. faecalis. The minimum inhibitory concentrations (MIC) of the antibiotic gentamicin (an aminoglycoside which inhibits protein synthesis) against the three species were determined. The MIC of S. pneumoniae was 9-12 ~g/ml, 48-50 ~g/ml for S. agalacliae, and 25 ~g/ml for S. faecalis. Though I labored to pinpoint a more exact concentration for each species, experimental variables resulted in the MIC ranges indicated above.

According to the Lancefield classification scheme, only S. faecalis is classified among the group D streptococci. For this experiment, however, I chose to focus on S. pneumoniae due to its clinical significance. This streptococcal species is the most common cause of bacterial pneumonia and is among the three leading causes of bacterial meningitis. Other diseases may include sinusitis and otitis media, endocarditis, arthritis, and peritonitis.

Through use of the Sonicor SC-1 00 sonicating bath maintained at an operating temperature of 37-38 C, sets of test tubes containing S. pneumoniae were incubated with a range of gentamicin concentrations (i.e. 11 ~g/ml down to 3 ~g/ml in series) with ultrasonication. Usually two identical sets of tubes were prepared for ultrasonication and three or more identical sets prepared at the same time were then incubated without sonication in another room at 37-38 C. Preparations which served as controls were utilized in all sets of reaction tubes. Consistency of technique in preparing and loading each tube was my goal. I experimented with two types of media: tryptic soy broth with the addition of dextrose (TSB/GLC), and brain-heart-infusion broth (BHI). S. pneumoniae grew especially well in the TSB/GLC media

Through visual assessment of the resulting turbidities (or absence thereof), both groups of reaction tubes (ultrasonicated and non-ultrasonicated) were compared with relative differences noted. I observed that the reaction tubes which were sonicated (for 24 hours) exhibited less turbidity than the corresponding non-ultrasonicated tubes. The differences were quite significant. I’m hesitant to make any bold declarations. The experimental data indicates that less growth occurs in the ultrasonicated samples. Whether or not the ultrasonication is serving to enhance the killing effect of gentamicin will, however, require further investigation. The potential exists that the ultrasonication itself (at a frequency of 67kHz) has a bactericidal influence on the cultures. The premise from previous research that this frequency has no detrimental effect will require further verification.