Here is a set of images to help you see what you should have observed from the Bacteriology Lab.
The first four images are what the blood agar plates from the four cases should have looked like:
Case 1 contains large somewhat beta-hemolytic colonies and small alpha-hemolytic colonies. There
are some larger colonies that might have appeared in the heavy regions of the streaking pattern, but
probably not in thE isolated area. The enlarged inset area shows the different colony types in more detail.
Case 2 contains small, white non-hemolytic colonies and small alpha-hemolytic colonies. The same
large colonies from Case 1 are present in the heavy streak area. The enlarged inset area shows the
different colony types in more detail.
Case 3 contains small heavily beta-hemolytic colonies and small, white non-hemolytic colonies. The
same large colonies from Case 1 are present in the heavy streak area. The enlarged inset area shows
the different colony types in more detail.
Case 4 contains small, white non-hemolytic colonies and small alpha-hemolytic colonies. The same
large colonies from Case 1 are present in the heavy streak area. The enlarged inset area shows the
different colony types in more detail.
Alpha hemolysis is shown more clearly in this blood agar plate of Streptococcus pneumoniae in pure
culture. The arrow in the inset shows that there is a very small colony surrounded by an olive green
discoloration and a thin, lighter halo of hemolysis.
All of the cases should have looked about the same on MacConkey Agar - about an equal mix of
Lac+ and Lac-colonies, but very few in number compared with the colonies seen on the blood agar
plates. The colonies observed from the MacConkey agar were "lost" in the heavy portion of the
blood agar. Why were they clearly visible on the MacConkey agar?
Staphylococcus aureus gram-positive cocci usually in bunches, but not always.
Staphylococcus epidermidis gram-positive cocci usually in bunches, but not always. Note that you
cannot distinguish the staphylococci by the Gram stain.
Streptococcus pyogenes (Group A strep) gram-positive cocci usually in chains, but not always.
Streptococcus pneumoniae gram-positive diplococci. Notice the characteristic elongated shape of
the diplococci.
Viridans streptococci gram-positive cocci usually in chains, but not always. Notice that these cannot
be differentiated from Streptococcus pyogenes by the Gram stain.
All of the gram-negative rods look the same.
The catalase test Note the bubbles in the lower positive test.
The coagulase test Note that the upper positive reaction has gelled, while the lower negative reaction
is liquid.
The optochin (P disk) test for identifying Streptococcus pneumoniae. Not the
zone of inhibition of growth for Streptococcus pneumoniae and the lack of
inhibition for viridans streptococci .
The bacitracin (A disk) test for identifying Streptococcus pyogenes. Note the zone of inhibition of
growth for Streptococcus pyogenes
The Phadebact test for rapid, antibody-based detection of Streptococcus pyogenes (Group A strep) .
The upper samples are the specific anti-A reagent, while the lower two samples are the negative
control reagent. The left 2 samples are a non-Group A strain, and the right 2 samples are Group A
strep. Note the agglutination reaction in the upper right and the lack of agglutination in the lower
right.
The citrate test is used in this lab exercise to differentiate E. coli from Klebsiella pneumoniae. In real
life there is much more involved than this simple test . The upper tube is a negative reaction with the
original green color remaining, while the lower tube is a positive reaction with the green having
changed to blue.