BIO 3305 Fall 2021
Lab 4 Part 1: Aerotolerance
Most microorganisms can survive within a range of environmental conditions, but not
surprisingly, tend to produce growth with the greatest density in the areas where conditions
are most favorable. One important source influencing microbial growth is oxygen. Some
organisms require oxygen for their survival, others are not affected by it, and some cannot even
survive in its presence. This ability or inability to live in the presence of oxygen is called
aerotolerance.
When growth media is prepared it is sterilized in the autoclave. This process not only
removes unwanted microbes, but also removes most of the free oxygen from the medium as
well. After the media is removed from the autoclave an allowed to cool the oxygen begins to
diffuse back in, forming a gradient of oxygen concentrations, ranging from aerobic at the top
(nearest to the source of oxygen) to anaerobic at the bottom. Because microorganisms
naturally proliferate where the oxygen concentration best suits their metabolic need, differing
degrees of population density will develop in the medium over time that can be used to visually
examine their aerotolerance.
Aerotolerance of microorganism:
Obligate (strict) aerobes: require oxygen for respiration, grow tat the top were oxygen
is most plentiful
Facultative anaerobes: grow in the presence or absence of oxygen. When oxygen is
available, they respire aerobically. When oxygen is not available, they either respire
anaerobically (reducing sulfur or nitrate instead of oxygen) or ferment an available
substrate.
Aerotolerant anaerobes: organisms that don’t require oxygen and are not adversely
affected by it, they will live uniformly throughout the medium. These organisms are
fermentative even in the presence of oxygen.
Microaerophiles: survive only in environments containing lower than atmospheric levels
of oxygen. Found in the upper to middle region of the medium. Capnophiles: can
survive only if CO2 levels are elevated
Obligate (strict) anaerobes: organisms for which a small amount of oxygen is level and
therefore will only be seen in the lower regions of the medium.
APPLICATION:
