Interpreting Microbiology Reports


The microbiology reports that follow are all from the same patient, same culture.



Step 1: The Gram stain

The process of Gram staining



S. aureus and P. aeruginosa in sputum


The first step in performing cultures is to do a Gram stain of the collected sample (not for blood/urine - that is cultured first, then stained).  Here is a report of a bronchial wash (BAL) Gram stain.  Specified are the date of culture (blocked), type of culture (bronch wash), procedure done (Gram stain), and results - few WBCs, no organisms seen.  A more exciting Gram stain may say "many WBCs, many Gram positive cocci in clusters", or something like that.

On rotation, report this as "on (date) a BAL was performed, Gram stain showed few WBCs and few organisms".



Step 2: Identification


Chemical tests to identify bacteria



A positive coagulase test



Next the bacteria are cultured and put through biochemical tests to determine their identification.  This takes a while longer than the Gram stain.  This is the same specimen for the same patient as above, reported a day later.  The BAL is growing a few Staphylococcus aureus and a few Pseudomonas aeruginosa.  The designation of "few" is a semi-quantitative description - number of bacteria and WBCs are often reported as rare, few, moderate, or many.  Here even though no bacteria were seen on the Gram stain, some did grow on culture.  If the "SPEC #" was not blocked out, we could see that it is the same as in the Gram stain.

On rotation, report this as "on (date) a BAL was performed which initially showed no bacteria but few WBCs on Gram stain.  It is growing few Staphylococcus aureus and few Pseudomonas aeruginosa."



Step 3: Susceptibilities

Disk-diffusion susceptibility tests for Pseudomonas aeruginosa


Tube dilution susceptibility testing



Finally the bacteria are tested for their susceptibility to various antibiotics.  Once this data is known, we can transition from empiric to definitive therapy if the team has decided that an infection is indeed present.  The lab sometimes tests bacteria against many different antibiotics that we would never actually use - ignore them.

On rotation, report this as "on (date) a BAL was performed which initially showed no bacteria but few WBCs on Gram stain.  It is growing few MRSA and few Pseudomonas aeruginosa, which is susceptible to all antipseudomonal drugs tested."

(Pseudomonas is never susceptible to ampicillin, amp/sulbactam, cefazolin, cefotetan, cetotaxime, and TMP/SMX, so there is no reason to list them)



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All written materials copyright 2005-6 Jason C. Gallagher

Questions?  Email jason.gallagher  AT