A Lab Report on Microbial Growth

Topics: Bacteria, Growth medium, Escherichia coli Pages: 8 (1680 words) Published: March 18, 2016
David Kennedy
Bio 210
Lab Report 1
Microbial Growth

Background Information:

This lab was conducted in order to understand basic differences among differential and selective media, while recognizing how each media is used to isolate and identify microorganisms (Wistreich, 2003). The first microorganism analyzed was Staphylococcus epidermidis. This organism is gram-positive, single celled, arranged in grape-like clusters, and cocci in shape (Bukhari, 2004). S. epidermidis is approximately 0.5 to 1.5 micrometers in diameter, it is a facultative anaerobe but grows best in aerobic conditions (Bukhari, 2004). Typical hosts of this species are humans and other warm-blooded animals, where a favorable temperature of 37℃ is usually maintained in the intestinal tract. S. epidermidis may cause urinary tract infections (UTIs) and infections associated with intravascular devices such as prosthetic heart valves, shunts, etc. (Bukhari, 2004).

The second microorganism that was analyzed during this procedure was Escherichia coli. E. coli is a gram-negative and bacillus (rod-shaped) bacterium, that thrives in an optimum temperature of 37℃ commonly found in the intestinal tract of humans and other mammals (Cappucino and Sherman, 2011).. It is a facultative anaerobe that is not normally pathogenic, but pathogenic strains cause UTI’s and bladder infections (SCCC, 2013).

Another bacteria that was observed, was Proteus mirabilis. This microorganism is gram-negative and rod shaped. P. mirabilis is motile and “swarms” towards nutrients such as maltose (Murphy, 2004). It is a mesophile, which lives in an optimal temperature of 37℃. P. mirabilis is able to elongate itself and secrete a polysaccharide for motility on items such as medical equipment (Murphy, 2004). This organism is found in the human gastrointestinal tract, but can cause infections when in contact with the urinary tract, wounds, or lungs (Murphy, 2004).

The fourth bacterium that was analyzed in this experiment was Pseudomonas fluorescens. This gram-negative rod shaped bacterium has a distinct yellow color that glows under fluorescent light (Montie, 2011). It normally inhabits soil, plants, and water surfaces, but can cause respiratory tract infections, cystic fibrosis, pneumonia, and infections to burn patient wounds (Montie, 2011). The optimum growth temperature of P. fluorescens is between 25-30℃ (Montie, 2011).

As stated earlier, selective and differential media were used to further understanding of isolating and identifying microorganisms. Selective media allows microbial growth of target organisms, while preventing or slowing the growth of other microorganisms, based on nutritional content (Wistreich, 2003). Differential media does not retard the growth of any one organism, but the nutritional composition of the media causes certain microorganisms to grow differently (i.e. different colors) than other microorganisms (Wistreich, 2003).

Another important way to distinguish the differences between microorganisms, is by analyzing differences and similarities in metabolism. The oxidase test is a useful procedure to identify oxidase-negative gram-negative enteric bacterial rods from the also gram-negative rods, Pseudomonas and Aeromonas (Wistreich, 2003). The oxidase test detects the presence of cytochrome c, which is an enzyme that assists an organism with utilizing oxygen through aerobic respiration (Wistreich, 2003). The catalase test identifies if catalase is present in a bacterial sample. Catalase is an enzyme found in aerobic microbes that catalyzes hydrogen peroxide, and a positive test reveals oxygen bubbles forming while a negative results yields no bubbles as the bacteria does not contain catalase (Wistreich, 2003).

Selective and Differential Media:
Each half lab bench acquired two plates of each agar: Blood Agar (BA), MacConkey Agar (MAC), Mannitol Salt Agar (MSA), Hektoen Agar (HEK), and Trypticase Soy Agar...
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