SYSTEMATIC ANALYSIS OF ABC TRANSPORTERS INSTREPTOCOCCUS SANGUINIS

The bacterium Streptococcus sanguinis is a primary member of the human oral microflora and also has been recognized as a key player in the bacterial colonization of the mouth. It is considered the most common viridians streptococcal species implicated in infective endocarditis. In all kingdoms of life, ATP binding cassette (ABC) transporters are essential to many cellular functions. Sequencing of the SK36 genome provided the opportunity to study ABC transporter mutants and their relationship with acidity of the oral environment. Despite numerous studies that have focused on carbohydrate uptake systems in closely related streptococcal species such as S. mutans, S. pneumonia and S. pyogenes, the mechanism of the response of these ABC transporters to acidic conditions in S. sanguinis is still unknown. The capability of S. sanguinis to adapt in these harsh environments suggests this bacterium is capable of responding to various environmental stimuli. The purpose of this study was to examine ABC mutants to identify functions that contribute to acid tolerance in S. sanguinis. This study demonstrates that two acid-sensitive mutant genes, SSA_1507 and SSA_1508, identify genes involved in acid tolerance. The two mutants grew on different sugars and none of them showed a defect in sugar utilization at acid pH. We couldn’t recognize any significant differences in sugar uptake for the two acid sensitive mutants or in mutants of their neighboring genes. Thus, the observed acid sensitivity is not due to a failure to take up any of the common sugars tested. The cytoplasmic pH of S. sanguinis was studied with the fluorescent pH indicator (BCECF) and SK36 was observed to have a wider pH range than either of the two acid-sensitive mutants SSA_1507 or SSA_1508. In these two mutants, intracellular pH was not as well maintained. At all pH values tested, the mutants displayed a lower intracellular pH than the wild type. These observations indicate that the cell membrane of these two mutants is unable to protect the interior components from adverse effects of higher pH values and lower pH values, and prove that these two mutant genes SSA_1507 and SSA_1508 are unable to grow in lower pH values. These results support a role for these ABC transporters in proton pump or export and indicate that the mutants are less able to pump out protons.