Staphylococcus and Streptococcus - Lab Report Example

Staphylococcus/Streptococcus Biology Lab Report 7 December Identification of microorganisms is useful in various spheres of microbiology. This lab aimed at isolating and identifying bacteria from the human body. Swabs were obtained from the skin and throat and plated on sheep blood agar. Thereafter, a series of biochemical tests were performed leading to the identification of three bacteria species namely Staphylococcus aureus, Staphylococcus epidermidis and Staphylococcus saprophyticus. It was concluded that biochemical tests are useful tools for the identification of unknown microorganisms. Introduction The identification of microorganisms is useful in many spheres of microbiology. For example, in food microbiology, it aids in the identification of microbes that cause food spoilage. In medical microbiology, it helps in the correct diagnosis of microbial disease hence enabling the correct treatment of diseases. It also helps in the characterization of infectious microbes. The three main methods of microbial identification the use of morphological, biochemical and molecular techniques (Soni, 2007). Morphological methods begin with the isolation of microbes using differential and selective media followed by the observation of the physical features of the microbes under a microscope. Biochemical methods entail the use of physiological attributes such as the utilization of certain nutrients and production of specific enzymes. Molecular methods involve the isolation and sequencing of DNA from the microorganisms. Microbes can be classified into various groups depending on their morphological and biochemical features. The Lancefield grouping is a method of clustering catalase-negative, coagulase-negative bacteria according to the carbohydrate structure of bacterial antigens present on their cell walls (Cheville, 2009). The aim of this lab was to isolate naturally-occurring bacteria from the human body and identify them using biochemical tests. It was hypothesized that biochemical tests would help in the identification of microbes found in the human body. Materials and Methods A petri dish containing 1 TSA 5% sheep blood agar was divided in half. A sterile swab was used to swab the throat and skin. A streak inoculation was performed for each sample on each section of the plate, which was incubated for 24 hours at 37 oC. The resultant colonies were then tested for Gram staining and the presence of alpha, beta, and gamma hemolytic bacteria. This test was performed by observing the presence or absence of red blood cell hemolysis on sheep blood agar. The next test that was performed was the catalase test to determine the production of enzyme catalase. A small amount of the colony was placed on a glass slide after which a few drops of catalase reagent was dropped on the bacteria. The production of bubble indicated a positive test for catalase. The catalase-positive samples were then tested for bacitracin sensitivity by incubating the colonies in four sections of a blood agar plate containing bacitracin discs for 24 hours at 37 oC. A coagulase test was then performed following the observation of bacitracin resistance in the bacteria. This test was performed to detect the production of enzyme coagulase by adding a loop full of the bacteria to tubes containing rabbit plasma and incubating for 24 hours at 37 oC. The coagulase negative samples were further tested for novobiocin susceptibility by incubating the bacteria in plates containing novobiocin antibiotic disks at 37 oC for 24 hours. Novobiocin test is usually used to distinguish between S. epidermidis and S. saprophyticus. The findings of the novobiocin susceptibility test enabled the identification of the microorganisms as indicated in table 1 in the results section. Results The isolated bacteria appeared as clusters as opposed to chains. The presence of catalase in the bacteria was indicated by the immediate effervescence. Positive coagulase test was indicated by the solidifying of the liquid in the tube while negative coagulase test was shown by the liquid remaining as a liquid. Susceptibility and resistance to novobiocin was determined by measuring the zones of inhibition around the antibiotic discs. The outcomes of the tests are summarized in table 1. Table 1: Biochemical tests for bacteria isolated from the skin and throat Test Staphylococcus aureus Staphylococcus epidermidis Staphylococcus saprophyticus Morphology/colony color Gold to yellow White White to yellow Gram staining + + + Hemolysis Beta None None Catalase + + + Bacitracin Resistant Resistant Resistant Coagulase - - Novobiocin Susceptible Resistant The findings in table 1 were then used to create a flow chart identifying the bacteria as shown in figure 1. Figure 1: Flow chart identifying the isolated bacteria The presence of clusters of cells, positive Gram staining and positive catalase results meant that the bacteria were Staphylococcus. The resistance to bacitracin also indicated that the bacteria were S. aureus, S. epidermidis and S. saprophyticus. A positive coagulase test identified S. aureus from S. epidermidis and S. saprophyticus. Susceptibility to novobiocin ultimately distinguished S. epidermidis from S. saprophyticus. Discussion The catalase test was vital in the differentiation of catalase-positive Micrococcaceae from catalase-negative Streptococcaceae. The bacitracin test is helpful for distinguishing β-hemolytic Group A streptococci from other β-hemolytic streptococci (Batt & Tortorello, 2014). This separation is vital because important because most streptococcal diseases are caused by Group A streptococci. The bacitracin test can also be used to differentiate the bacitracin-resistant Staphylococcus from the bacitracin-susceptible Micrococcus. Conversely, coagulase test is useful in differentiating Staphylococcus aureus from other species of Staphylococcus. S. aureus produces coagulase, which is an enzyme that is responsible for the conversion of soluble fibrinogen in plasma to an insoluble substance known as fibrin (Batt & Tortorello, 2014). In the experiment, the tube coagulase test was carried out to detect free coagulase. Another variant of the same test known as slide coagulase test is often done to detect bound coagulase, which is also referred to as clumping factor. The behavior of various microorganisms in the presence of different chemicals enabled the identification of bacterial isolates of the genus Staphylococcus up to species level. Therefore, it was concluded that biochemical tests are accurate and reliable methods of identifying bacterial colonies. Additionally, the response to various biochemical agents is useful in categorizing bacteria. References Batt, C. A. & Tortorello, M-L. (2014). Encyclopedia of food microbiology (2nd ed.). San Diego: Academic Press. Cheville, N. F. (2009). Ultrastructural pathology: The comparative cellular basis of disease. USA: John Wiley & Sons. Soni, S. K. (2007). Microbes: A source of energy for 21st century. New Delhi: New India Publishing. Read More