5 May 2018
Species 1) Serratia marcescens
Species 2) Serratia liquefaciens
Species 3) Serratia proteamaculans
Unknown Identification for Serratia marcescens
The study of microbiology doesn’t simply require just an educational understanding of the microscopic world, but also a tangible comprehension of lab techniques and most importantly, procedures to identify unknown organisms. The unknown for this experiment was #15, and the gram negative unknown was identified to be Serratia marcescens.
The differentiation of Serratia species pointed towards several species such as Serratia liquefaciens (#2) and Serratia proteamaculans (#3) due to similar testing results, but mainly towards Serratia marcescens. The unidentified Serratia species is a facultative anaerobe, denoting that it could possibly switch to fermentation to grow in the absence of oxygen (anaerobic) or make ATP in the presence of oxygen (aerobic) by aerobic respiration. The bacterium uses fermentation as its metabolic capacity and tests positive on glucose, mannose, and sucrose with gas production, but negative for lactose fermentation. Secondly, the facultative anaerobic within the genus Serratia tested positive for gelatinase which causes the hydrolyzes gelatin by using it as a food source. This protein digesting ability presumably converts the protein into polypeptides and amino acids, causing the gelatin cultures of these species to not solidify under cold conditions therefore allowing it to exist in terrestrial environments such as fresh water or soil. Based on their phenotypic profile shaped by these results, the genus can be divided into three species S. liquefaciens, S. proteamaculans, and S. marcescens. However the distinguishing factor was present within the pinkish-red pigment called prodigiosin that S. marcescens is known for, which other species like S. liquefaciens (#2) and S. proteamaculans (#3) lacked.
Beyond using table 5.34, there were several other biochemical tests by research groups that helped confirm the identity of S. marcescens. S. marcescens tests positive for VP test, implying that the organism transforms pyruvate into acetoin using the butylene glycol pathway. Likewise, S. marcescens tested negative for the MR test due to its production of acetoin which neutralizes the acidic PH. S. marcescens also uses nitrate as the terminal electron acceptor and tested positive.
The series of tests performed on the unknown and table of identification of bacterial genus on pg. 99 along with differentiation of Serratia species on Table 5.2 of pg. 222 led to S. marcescens as the only possible candidate. This result was plausible because Serratia species are sweeping epidemic pathogens distributing problems nationally. S. marcescens is commonly found in nutrient rich environments such as animals, plants, air, water, soil, or animals, and in fact, this corresponds to the source of the unknown which was in fact, soil.