Cultural Characteristics: Growth on
Slants and Broths
Growth of bacterial cultures on agar
slants and in broths can provide us with useful information concerning
motility, pigmentation and oxygen requirements. While there is variation
even among individual strains of the same species, some characteristics
are distinctive, thus can aid in the beginning steps of identification.
In general, bacterial growth on slants
ranges from even (following the line of the original streak), to
irregular (slight spreading from the original line), to spreading
(the organisms cover the entire surface of the slant). Nonmotile
bacteria such as cocci usually produce even slants when viewed after 24-48
hours incubation, while slightly motile to motile species produce irregular
to spreading growth.
All samples were grown on trypticase
soy agar (TSA) for 48 hours at 37o C. Click on each image
to see a larger view.
is a slant of Staphylococcus aureus. Note the even pattern of growth
which follows the line of inoculation. The wider portion at the bottom
is due to the presence of a small amount of condensation.
This is a slant of Bacillus subtilis. Note the spreading pattern
When bacteria are grown in broths such as trypticase soy broth (TSB), they
may exhibit patterns of growth ranging from a sediment at the bottom
of the tube, turbid growth throughout the tube, or a pellicle
(thick growth at the top of the tube). Pellicle formation is sometimes
due to a affinity for oxygen, but may also be the result of hydrophobic
compounds present in the cell wall or the general formation of dry, light
colonies. Also, if an organism produces and releases soluble pigments,
these will spread into the broth and change its color. Here are two
examples of growth patterns in broth after 48 hours incubation at 37o
This broth contains the acid-fast species Mycobacterium smegmatis.
Note the pellicle on the surface of the broth which forms due to the high
concentration of hydrophobic mycolic acids embedded in the cell wall of
This broth contains Serratia marcescens, a gram-negative rod.
Observe the turbid appearence of the broth and the red color present in
both the sediment and pellicle, which is the result of the nonsoluble pigment
prodigiosin produced by this bacterium.