The Optochin Susceptibility Test: A Diagnostic Tool for Streptococcus pneumoniae
The Optochin Susceptibility Test is a simple, reliable, and widely used biochemical test in clinical and diagnostic microbiology laboratories. Its primary purpose is the presumptive differentiation and identification of *Streptococcus pneumoniae* (pneumococci) from other alpha-hemolytic streptococci, which are often grouped as viridans streptococci. Alpha-hemolytic streptococci cause partial clearing of blood and greenish discoloration on Blood Agar plates, making their distinction difficult based on colony morphology alone. Optochin susceptibility is a crucial differential characteristic, allowing for the rapid diagnosis and management of pneumococcal infections, which can manifest as pneumonia, meningitis, and septicemia. Historically, the distinction between these groups was often confirmed by the bile solubility test, but the optochin test is now favored for its simplicity and efficiency.
Principle of Optochin Susceptibility
The core principle of the optochin test relies on the selective inhibitory and lytic effect of the chemical agent optochin. Optochin is the common name for ethylhydrocupreine hydrochloride, a derivative of quinine. This chemical is highly water-soluble and diffuses readily into the agar medium surrounding the impregnated disc. Optochin acts as a selective inhibitor by specifically targeting a vital cellular component in *Streptococcus pneumoniae*: the transmembrane F0F1 ATPase. This enzyme is crucial for ATP synthesis and membrane function.
For *S. pneumoniae*, optochin selectively binds to and interferes with the F0F1 ATPase, which in turn compromises the integrity of the bacterial cell membrane. This binding causes the bacterium to lyse, or break down, due to changes in surface tension, effectively killing or inhibiting the growth of the organism. This lytic effect is pronounced even at the very low concentration (5 µg) found in the test disc. When this inhibition occurs, a visible, clear area where bacterial growth is prevented—known as a “zone of inhibition”—is formed around the optochin disc on the seeded agar plate.
In stark contrast, other alpha-hemolytic streptococcal species, most notably those belonging to the viridans streptococci group (such as *Streptococcus mitis* and *Streptococcus sanguinis*), typically lack this extreme sensitivity. They are considered optochin-resistant and will generally grow right up to the margin of the optochin disc. While some viridans streptococci may show a slight susceptibility or a very small zone of inhibition, this is generally less than the standard cutoff diameter. This fundamental difference in sensitivity forms the basis for presumptively identifying *S. pneumoniae* from other morphologically similar bacteria.
Procedure of the Optochin Susceptibility Test
The optochin susceptibility test is performed using a modified version of the standard disk diffusion method. Proper technique, the correct culture medium, and appropriate incubation conditions are essential to prevent false results.
1. **Medium Preparation:** The recommended medium is a 5% Sheep Blood Agar plate, as media other than TSA-5% sheep blood agar are not advised and can lead to indeterminate results. This medium provides the necessary blood components for the alpha-hemolytic growth to be observed.
2. **Inoculation:** An inoculum is prepared either by selecting three to four well-isolated colonies of the alpha-hemolytic organism to be tested or by preparing a bacterial suspension. The isolate is streaked onto one-half of the blood agar plate to achieve a confluent lawn of growth.
3. **Disc Application:** Using sterile forceps, an optochin disc, typically 6 mm in diameter and impregnated with 5 µg of ethylhydrocupreine hydrochloride, is placed onto the inoculated surface. The disc must be gently pressed with the sterile forceps or loop to ensure it adheres firmly to the agar. A note for some procedures is that a drop of sterile distilled water can be placed on the disc to enhance the optochin diffusion into the medium, which can lead to larger zones of inhibition.
4. **Incubation:** The plate is inverted (lid down) and incubated at 35-37°C for a standard duration of 18-24 hours. A critical requirement is incubation in a 5-10% CO2-enriched environment. This is paramount because *S. pneumoniae* strains may grow poorly in ambient air conditions, and the CO2 enrichment promotes optimal growth and the development of a clearly defined and larger zone of inhibition for sensitive strains.
5. **Quality Control:** Every new batch of optochin discs must be checked for potency using a known optochin-sensitive positive control strain (*S. pneumoniae* ATCC 49619) and a known optochin-resistant negative control strain (*S. mitis* ATCC 49456) to ensure the reliability of the test results.
Interpretation of Optochin Test Results
Following incubation, the plate is examined for the presence and size of a clear zone of inhibition around the optochin disc. The zone diameter is measured in millimeters, including the diameter of the disc, using a ruler or caliper. Results are reported as sensitive (susceptible) or resistant.
1. **Sensitive (Presumptively Positive):** A zone of inhibition that is 14 mm or greater in diameter around the 6 mm disc indicates that the test organism is optochin-sensitive and is presumptively identified as *Streptococcus pneumoniae*. If a 10 mm disc is used, a zone of 16 mm or greater is considered sensitive.
2. **Resistant (Negative):** No zone of inhibition, or a zone of inhibition that is less than 14 mm in diameter, indicates the test organism is optochin-resistant and is therefore presumptively identified as a non-pneumococcal alpha-hemolytic streptococcus (viridans streptococci).
3. **Equivocal or Indeterminate Results:** Any zone of inhibition between 6 mm and 14 mm is considered equivocal. Such organisms, which include those with borderline diameters, should not be definitively identified as *S. pneumoniae* solely on this test. In these cases, the isolate must be subjected to a confirmatory test, traditionally the bile solubility test. The organism is presumptively identified as a pneumococcus only if it is found to be bile-soluble.
Limitations and Clinical Significance
Despite its high utility, the optochin susceptibility test is a presumptive identification method, and laboratories must be aware of its limitations. Rare isolates of *S. pneumoniae* that are optochin-resistant have been documented, often linked to point mutations in the gene coding for the ATPase target. Conversely, some non-pneumococcal alpha-hemolytic streptococci may show minor, yet variable, susceptibility. For this reason, isolates with equivocal results necessitate an additional test like bile solubility for full confirmation. The test’s overall sensitivity and simplicity, however, make it an indispensable tool for rapid clinical laboratory diagnostics, allowing for the timely differentiation of the pathogenic *S. pneumoniae* from the ubiquitous and often commensal viridans streptococci, which is a crucial first step in determining appropriate patient treatment.