Introduction

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        The bacteria Serratia marcescens produces an enzyme called prodigiosin. When grown at room temperature, the enzyme produces a bright red color that causes red colonies. When grown at 37°C, the enzyme is denatured, and the bacteria display a cream color. This indicates that pigment-less bacteria are naturally cream colored and shows a phenotypic change in the bacteria. A phenotypic change is a change in the displayed traits of an organism, which may be caused by the environment, like temperature. A genotypic change can also change the phenotype and is due to a change in the organism’s DNA. Pigment-less bacteria can be produced through a mutation in the gene, or a genotypic change, that codes for the prodigiosin enzyme. The mutation can be caused by UV light, which creates thymine dimers. Thymine dimers are bonds formed between adjacent thymine bases in DNA, which can be repaired using photorepair. In photorepair, bacteria can use light to repair the damage. However, this reparation can cause mutations, or permanent genetic changes to the bacterial DNA. This can lead to several possible outcomes. A high dosage of UV radiation could completely kill an entire plate of bacteria. If the bacteria do survive, the mutations caused by the UV radiation could eventually kill it and no colonies would form. If a mutation occurs in the gene that codes for the enzyme that causes a red pigment, photorepair can fully repair the DNA and the bacteria can reproduce as normal to make red colonies. However, if photorepair does not repair the mutation properly, pigment production can be inhibited, causing cream colored bacteria. If the bacteria pass on the mutated gene, cream colonies will be formed, and the mutation can be determined to be heritable. Further studies have been done to show evolutionary benefits of a mutated pigment gene. They suggest cream-colored bacteria are more resistant to further UV radiation than red bacteria (Hanks et al. 1971). It is hypothesized that if the bacteria are subjected to a high UV dosage and incubated in the dark, permanent but heritable damage will cause colonies of cream-colored bacteria to form. If the bacteria are subjected to a high UV dosage but then incubated in the light, the bacteria will photorepair and produce colonies of red-colored bacteria. Bacteria subjected to low or no UV radiation will only produce red colonies, regardless of incubation in light or dark (Shibai et al 2017).