Bacteriology for CLS and MLT

Over the years, countless number of CLSs confided with me that when it comes to Microbiology, they don’t know where to begin, they’ve forgotten everything. For those CLSs, here’s a piece of knowledge from someone who had spent the majority of her CLS career in working and teaching in microbiology…when a culture plate is in front of you, it is usually wise to take a deep breath (to prevent hyperventilation), and ask yourself the first question, what’s the source of this culture plate? Is it urine, throat, sputum, wound, stool, blood or other sterile fluid (knowing the source will narrow your choices of possible organisms)? Once you know what you are dealing with, then you can turn the first plate over and ask yourself the second question, what kind of a culture plate am I holding in my hand? Is it a non-selective media such as BAP or CAP where almost everything will grow? Is it a selective media such as MAC, PEA or CNA wherein only one type of bacteria will grow and inhibit the other type? Or is it a selective and differential media such as MAC, SS or HE wherein it will selectively grow one type of bacteria as well as allowing differentiation between bacteria of the same type by color or morphology? You’re almost there…(take another deep breath) you now compare what is growing in your non-selective media with your selective or selective and differential media and Eureka! You’re dealing with a gram positive or a gram negative bacteria (whichever one is not growing on the selective or selective and differential media gets eliminated!)! Oh, You’re still not sure? Well, ah, walk discreetly to the gram staining area and gram stain a colony or two (make sure you’re picking the same colony morphology!) and confirm your suspicion. Now that you’re confident, now what? If you’re dealing with gram positive, examine the colonies in the culture, does it look like cocci or rod? Still not sure? Try to compare the growth of your culture against the pictures in this manual or check the list of common organisms associated with a source, Table 2 in this manual (this will at least give you a clue on the most common organisms based on source) or lastly, uhrmm, your gram stain? Alright, so let’s say we’re dealing with cocci, is it Staph, Strep or Enterococcus? Not sure? No problem, until your eyes gets used to it again, whip out your catalase reagent (a must in your labcoat pocket), bubbling? Staph! Then just follow your laboratory’s SOP on staph identification or you can refer to the figure 4 (Gram Positive Cocci Identification Chart) of this manual or lastly, if you’re using an automated identification system, just rack and load ‘em.