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.

This 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 of growth.


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 C:

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 species.

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.


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