Optochin sensitivity refers to a bacterium’s susceptibility to the inhibitory effects of a specific chemical compound called optochin. This characteristic serves as a diagnostic tool in laboratory settings, helping to identify particular types of bacteria.
Understanding Optochin Sensitivity
Optochin is a chemical compound derived from quinine. In microbiology, “sensitivity” indicates that a bacterium’s growth is inhibited by the presence of optochin. This means that when exposed to optochin, sensitive bacteria cannot multiply effectively.
Optochin’s main use is in differentiating Streptococcus pneumoniae (pneumococcus) from other related bacteria, especially other alpha-hemolytic streptococci, such as those in the viridans group. It works by interfering with bacterial enzymes, causing the sensitive S. pneumoniae cells to lyse, or break open. While S. pneumoniae is typically sensitive to optochin, some strains have shown resistance, highlighting the importance of using this test in combination with other methods for complete identification.
Why Identifying Streptococcus pneumoniae Matters
Accurately identifying Streptococcus pneumoniae is important for patient care and public health. This bacterium commonly resides in the noses and throats of healthy individuals, but it can cause various infections, ranging from mild to life-threatening, particularly in young children, older people, and those with weakened immune systems. S. pneumoniae is a leading cause of community-acquired pneumonia, otitis media (middle ear infections), sinusitis, and more severe invasive diseases.
Invasive pneumococcal diseases occur when the bacteria spread to normally sterile body sites, causing meningitis (inflammation of the membranes surrounding the brain and spinal cord), bacteremia (bloodstream infection), and sepsis (a severe, life-threatening response to infection). Rapid and accurate identification of S. pneumoniae guides appropriate antibiotic choices and helps prevent the spread of infection. Misidentification could result in inappropriate treatment, potentially leading to prolonged illness, severe complications.
Performing and Interpreting the Test
The optochin sensitivity test is a laboratory procedure used to identify Streptococcus pneumoniae. The process begins by culturing the bacterial sample on a 5% sheep blood agar plate to obtain confluent growth, ideally from well-isolated colonies. A small filter paper disc impregnated with 5 micrograms of optochin is then placed onto the inoculated agar surface using sterile forceps, ensuring it adheres firmly. The plate is incubated at 35-37°C for 18 to 24 hours in an environment enriched with 5-10% carbon dioxide, as some S. pneumoniae strains may not grow well in ambient air.
After incubation, the plate is examined for a clear zone of inhibition around the optochin disc, which indicates that the bacterial growth has been inhibited. A zone of inhibition measuring 14 millimeters or greater in diameter around a 6-millimeter disc is considered a positive result, strongly suggesting the presence of Streptococcus pneumoniae. If no zone of inhibition appears, or if the zone is less than 14 millimeters, the bacterium is considered resistant or intermediately susceptible to optochin. In cases with intermediate zone sizes (e.g., between 6-14 mm), further testing, such as a bile solubility test, is often performed to confirm the identification of S. pneumoniae.