Microbiology

Author: Mikael Häggström, M.D.

Learning microbiology

Put substantial effort on identifying what is actually memorization-worthy before starting to memorize microbiology-related content. For example, have an idea of the main workflows in a typical microbiology lab, but do not waste much time memorizing bench tests (negative/positive for indole, lactose/sucrose fermentation etc. etc.). In everyday practice there are only a limited number of bacteria that are identified by such testing, whereas the rest are generally identified by multitarget assays and/or MALDI-TOF. For exams where the passing score is an average of multiple topics (including the American Board of Pathology Certification Exams), focus on more useful topics instead. Further information: Learning pathology

Example algorithm

Following is an example of a workup algorithm of possible bacterial infection in cases with no specifically requested targets (non-bacteria, mycobacteria etc.), with most common situations and agents seen in a New England community hospital setting. The grey box near top left shows a Venn diagram of what culture media are routinely used for various sources or purposes. The included pathways are the most common ones, but are not mandatory, as there are multiple exceptions to them.

Test question Proteus-looking culture Early Proteus-looking colonies The microbiology technician asks you to have a look at a recent wound culture in a patient whose previous blood culture showed gram positive bacteria. The blood agar (shown) grows early colonies of what mainly looks like Proteus (which is gram negative). Is it acceptable to take a sample for MALDI-TOF or multitargeted assay without further bench testing? Answer No. In this case, we want to particularly rule out gram positives in the culture, but Proteus tends to swarm all over blood agars and get mixed with other cultures, whereas the mentioned machines should receive pure cultures. In this case, one or more samples should be plated on an agar that inhibits gram negatives (such as CNA) in order to detect gram positives.

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