Non-spore forming Aerobic Gram Positive Bacilli
These are separated on the PowerPoint into catalase positive and catalase negative organisms. Catalase
is an enzyme that the bacteria may possess that gives them the ability to break down H2O2 into H2O
and CO2. We can visualize the presence of this enzyme by performing a catalase test. We introduce a
few colonies of bacteria into H2O2 and look for the rapid production of bubbles. We use this test to
help us identify bacteria so it is an important characteristic to look at.
Corynebacterium – Obviously, since these are in a PowerPoint titled “…. Gram positive bacilli” we expect
to see gram positive rods on gram stain. These are aerobic organisms and there are a lot of species in
this genus. A lot of the species that we see in the clinical lab are usually considered normal flora and
contaminants in patient specimens. The major pathogen in this group (and the one we really need to
learn about) is Corynebacterium diphtheriae. But let’s talk a minute about general characteristics of
Corynebacterium. They are divided into two groups. Lipophilic and nonlipophilic. What this means for
us is that come of the species will grow much better if lipid is added to the media. Others do not need
that additional lipid and will grow very well on standard media. It’s important for us to know what we
may be looking for when we inoculate culture media so we know what type of media to use. If we have
a lipophilic organism and it has been inoculated to standard media, it may take 48 hours or more for
growth to appear. Depending on your culture type, the plates may be discarded after 24-48 hours with
no growth. We could miss the organism. The gram stain of the rods is pretty characteristic.
Corynebacterium tends to present with curved and “club-shaped” rods on gram stain. This is referred to
as “diphtheroid”. When you get to the clinical lab, you will hear the micro techs say there are
“diphtheroids” present from time to time (especially in urine cultures). This is usually dismissed as
contamination and not worked up. They will know this from the gram stain.
Corynebacterium diphtheriae – causative agent of diphtheria. Do we see this in the US a lot? Nope.
Why not? Yep…. Because we have a vaccine for this. But we may see it some because immunity may
decrease or a child may not have gotten the vaccine. Various reasons. Humans are the natural host for
this organism. It is not normal flora. It is a pathogen. Diphtheria can present in two forms. Respiratory
or cutaneous. When I think of diphtheria, I think of the respiratory form but there is also a cutaneous
presentation. A very important thing to remember… just because you isolate Corynebacterium
diphtheriae from a patient specimen, does not mean that the patient has diphtheria. In order for the
organism to cause illness in the patient, the organism needs to possess the diphtheria toxin. The C.
diphtheriae is infected with a beta phage (bacterial virus) carrying the tox gene that causes the organism
to produce the toxin. The toxin works by disrupting protein synthesis in our cells. It is composed of two
fragments – A and B. The fragments work together to cause the effects. Once the toxin is exposed to
trypsin (in our cells) it is separated into its two separate fragments. Fragment B binds to the host cell
allowing fragment A to enter the cell and halt protein synthesis. In order to determine if an isolate is
infectious, we need to be able to observe the toxin presence. We can produce the toxin in the lab by
growing the organism in a media enriched with iron. Once the iron is depleted in the media (from
bacterial growth) the organism (if able) will produce the toxin. If you skip ahead to slide 25 we will talk
about how to test for the toxin. To determine the presence of the toxin (either from the initial specimen
isolate or from production in the lab) we can perform an immunodiffusion test called the Elek test. On a
culture plate containing a medium with a low iron content, the unknown organism, a positive control
(an organism known
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