The bacteriology section of the microbiology department is where pathogenic bacteria are cultured and identified, and their antibiotic sensitivities are tested.
The first step in bacterial identification is culturing (growing) the bacterium. The specimen is plated on agar-based media or inoculated in broth media or blood culture bottles. The bacteria grown on agar-based media will produce visible colonies that differ in size, shape, and color. For example, Staphylococcus aureus appears as medium-sized, yellow colonies while Streptococcus pneumoniae appears as small, green/gray colonies.
The second step is isolating a pure colony. There may be instances where normal bacteria can grow alongside the pathogenic bacteria. Therefore, staff must isolate a pure colony of the pathogenic bacterium for identification.
The third step is identifying the pure colony. Gram staining and biochemical testing are used to identify the bacterium. Gram staining is performed by staining the isolated colonies on a glass slide. Bacteria can stain pink, indicating they are Gram-negative, or stain purple, indicating they are Gram-positive. Biochemical testing is performed by incubating isolated colonies with different chemicals. The way the colonies react with the chemicals helps determine the bacterium's identity.
After identification, the final step is determining antibiotic sensitivity. Pure colonies are replated and allowed to interact with antibiotic-infused disks. An effective antibiotic will produce a bacteria-free ring around the disk, known as the zone of inhibition. This indicates that the bacteria may be sensitive to the antibiotic.
In the modern bacteriology department, biochemical testing and antibiotic sensitivity are automated and performed simultaneously. While this has made testing more efficient, these automated instruments still rely on traditional techniques and professional review. The future of microbiology instrumentation has shifted from visual examination to high-tech alternatives, including matrix-assisted laser desorption ionization mass spectrometry (MALDI-TOF) and polymerase chain reaction (PCR). MALDI-TOF is a type of mass spectrometry that identifies the bacterium based on its mass and desorption properties. PCR replicates a bacterium's DNA sequence, which can be matched to known sequences of bacterial DNA to diagnose the infection. These instruments provide more detailed and possibly more reliable results in a quicker turnaround time.
