Microbial Identification Laboratory Report

Introduction

The process of identifying unknown bacteria is of paramount importance in both clinical and research settings. Accurate identification is crucial for healthcare providers to administer appropriate treatment. Furthermore, bacterial identification enables testing for antibiotic sensitivity, aids in the discovery of new microorganisms, and assists in tracking infectious diseases within populations (LearnSmart Labs, 2017).

This lab focuses on three main approaches to microbial identification: genetic analyses, immunologic methods, and phenotypic testing. Genetic analyses can differentiate between bacterial strains based on minute differences in DNA sequences.

However, genetic analysis can be prone to contamination and is expensive. Immunologic tests provide insights into sensitivity and antibodies of infectious agents but can yield false-positive results and require the presence of sufficient antibodies in the patient's response to the microbe. Phenotypic testing, which involves direct observation under a microscope, is a common approach to identify bacteria but is time-consuming and necessitates the cultivation of the organism (LearnSmart Lab, 2017).

In this lab report, we will discuss the identification of an unknown bacterium isolated from a patient's stool sample.

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The identification process involved a series of differential and diagnostic tests to determine the nature of the bacterium.

Materials and Methods

Four differentiating tests were performed on the stool sample obtained from the patient. The initial step involved culturing the sample on an agar plate, followed by the creation of a stock culture after 24 hours of incubation. A standard gram stain technique was employed, revealing numerous gram-negative bacilli (Learnsmart Labs, 2017).

Subsequently, the isolated sample underwent several differential and diagnostic tests based on a dichotomous key:

1. Oxidase Test: A sterile Q-tip was used to sample an isolated colony of bacteria.
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   Two drops of oxidase reagent were added to the cotton swab tip, and any color change was observed after 10-30 seconds.

2. MacConkey Agar Test: Inoculation of MacConkey agar medium was performed using aseptic technique. The plate was incubated at 37 degrees Celsius for 24 hours, and colony growth and color change were observed.
3. Hydrogen Sulfide Test (HS Test): A TSI agar slant tube was inoculated with the unknown organism using aseptic technique. The tube was incubated at 37°C for 24 hours, and color changes were recorded.
4. Urease Test: Urea medium was used to detect the presence of the urease enzyme. Inoculation was conducted similarly to the HS test, and incubation proceeded at 37°C for 24 hours. Any color change in the tube was documented.

Results

The results of the four tests conducted on the unknown bacterium are summarized below:

Based on the results of these tests, the bacterium was identified as Salmonella typhimurium.

Discussion

The main objective of this lab experiment was to identify an unknown bacterium isolated from a stool sample obtained from a patient. Accurate identification is crucial as it guides healthcare providers in selecting the most effective antibiotic treatment. An incorrect identification could lead to ineffective treatment and adverse consequences for the patient.

Salmonellae, like other Gram-negative bacilli, possess a complex lipopolysaccharide (LPS) structure within their cell envelope. This LPS structure, consisting of an outer O-polysaccharide coat, a middle R core, and an inner lipid A coat, plays a significant role in determining virulence. The presence or absence of complete O-sugar repeat units can influence the virulence of salmonellae. Smooth strains with a full complement of O-sugar repeat units are typically more virulent than rough strains lacking complete sequences. The endotoxin component of the cell wall, including the LPS, may contribute to the pathogenesis of various clinical manifestations of Gram-negative infections, leading to symptoms such as fever, complement activation, and clotting system activation.

Diagnosing a salmonella infection involves bacteriological isolation of the organisms from appropriate clinical specimens. Laboratory identification of the genus Salmonella relies on biochemical tests and serologic typing. Various agar media, including MacConkey agar, are used to culture specimens, followed by biochemical testing using triple sugar iron agar and lysine-iron agar to make a presumptive identification. Rapid systems that test multiple biochemical parameters simultaneously have simplified the process. Confirmation is achieved through serologic analysis of O and H antigens using polyvalent and specific antisera.

In conclusion, accurate identification of bacteria is crucial for guiding appropriate antibiotic treatment. In this lab, we successfully identified the unknown bacterium from a patient's stool sample as Salmonella typhimurium based on the results of several key tests. This information aids in providing effective medical care and contributes to the responsible use of antibiotics.