Question

A microbiology instructor prepares cultures for a gram-staining laboratory by inoculating growth medium with a gram-positive coccus (non motile) and a gram-negative rod (motile). The goal is to demonstrate staining of a mixed culture of gram negative and gram positive cells. The flask in incubated at 35 C for 24 hours without aeration or shaking. A sample is stained and reveals only gram-negative rods. Both cultures are known facultative anaerobes. Give a likely reason for success of the gram-negative rod. Assume that the cultures have comparable intrinsic growth rates.

Answer 1

Based on oxygen requirements the microorganisms are categorized as aerobic, anaerobic, facultative anaerobic and microaerophilic etc. The aerobic organisms strictly grow only in presence of oxygen. The anaerobic organisms strictly grow only in absence of oxygen and any traces of oxygen is also harmful to them.

The facultative anaerobes can survive in presence of oxygen but can better grow in absence of oxygen. These organisms can better utilize any electron acceptor other than oxygen and grow. When the mixed culture of facultative anaerobic gram negative bacilli and gram positive cocci are grown, only the gram negative bacilli have succeeded and exhibited the growth.

This could be due to gram negative bacilli utilizing a suitable electron acceptor other than O₂ to carry out the anaerobic metabolism. On the other hand, the gram positive cocci could not utilize the electron acceptor other than O₂ efficiently and therefore could not exhibit the growth. Thus, the sample stained revealed only the presence of gram negative bacilli but not the gram positive cocci.