BIO 205 Microbiology Lab Report

Abstract

This BIO 205 microbiology lab report presents a comprehensive bacterial culture analysis and identification study. The experiment involved isolation of bacterial samples from environmental sources, followed by systematic identification using morphological, biochemical, and antimicrobial susceptibility testing methods. Results indicated the presence of Staphylococcus aureus and Escherichia coli, confirmed through Gram staining, catalase testing, and biochemical assays. This report demonstrates proficiency in aseptic technique, bacterial culture methods, and scientific documentation.

Introduction and Objectives

Bacterial identification is a fundamental skill in microbiology, essential for clinical diagnostics, environmental monitoring, and research applications. This laboratory exercise aimed to develop competency in:

• Aseptic Technique: Proper sterile handling of cultures and media to prevent contamination
• Culture Methods: Isolation and maintenance of pure bacterial cultures using streak plate technique
• Microscopy Skills: Gram staining procedure and microscopic observation of bacterial morphology
• Biochemical Testing: Application of diagnostic tests for bacterial classification and identification
• Antimicrobial Analysis: Evaluation of bacterial susceptibility to common antibiotics

Materials and Methods

Bacterial Isolation and Culture:

• Environmental samples collected using sterile swabs from designated surfaces
• Samples inoculated onto nutrient agar plates using streak plate method
• Plates incubated at 37°C for 24-48 hours to allow colony development
• Isolated colonies sub-cultured to obtain pure cultures for identification
• Colony morphology documented including size, shape, color, and texture

Gram Staining Procedure

Microscopic Analysis:

• Bacterial smears prepared on glass slides and heat-fixed
• Crystal violet applied as primary stain for 1 minute
• Gram's iodine mordant applied to fix the stain
• Decolorization with 95% ethanol for 10-20 seconds
• Safranin counterstain applied to visualize Gram-negative bacteria
• Slides examined under oil immersion (1000x magnification)

Biochemical Testing

Diagnostic Tests Performed:

• Catalase Test: Detection of catalase enzyme using hydrogen peroxide (positive = bubbling)
• Oxidase Test: Identification of cytochrome c oxidase using oxidase reagent
• Coagulase Test: Detection of coagulase enzyme for Staphylococcus aureus identification
• IMViC Tests: Indole, methyl red, Voges-Proskauer, and citrate tests for Enterobacteriaceae
• Sugar Fermentation: Glucose, lactose, and sucrose fermentation patterns observed

Antimicrobial Susceptibility Testing

Kirby-Bauer Disk Diffusion Method:

• Bacterial suspension prepared to 0.5 McFarland standard turbidity
• Mueller-Hinton agar plates inoculated using sterile swab technique
• Antibiotic disks applied: penicillin, ampicillin, tetracycline, erythromycin, gentamicin
• Plates incubated at 37°C for 16-18 hours
• Zones of inhibition measured and compared to CLSI standards
• Results interpreted as susceptible, intermediate, or resistant

Results and Observations

Bacterial Identification Findings:

• Isolate A: Gram-positive cocci in clusters, catalase positive, coagulase positive → Staphylococcus aureus
• Isolate B: Gram-negative rods, catalase positive, indole positive, lactose fermenter → Escherichia coli
• Colony Morphology: S. aureus showed golden-yellow pigmentation; E. coli displayed pink colonies on MacConkey agar
• Antibiotic Susceptibility: S. aureus resistant to penicillin, susceptible to gentamicin; E. coli susceptible to all tested antibiotics
• Growth Characteristics: Both organisms showed optimal growth at 37°C with visible colonies after 24 hours

Discussion and Analysis

Interpretation of Results:

• Successful identification of two clinically significant bacterial species using standard microbiological methods
• Gram staining results consistent with known morphological characteristics of identified organisms
• Biochemical test patterns matched expected profiles for S. aureus and E. coli
• Antibiotic resistance in S. aureus highlights importance of susceptibility testing in clinical settings
• Proper aseptic technique maintained throughout experiment, preventing cross-contamination
• Results demonstrate competency in fundamental microbiology laboratory skills

Conclusion

This BIO 205 microbiology lab report successfully demonstrates proficiency in bacterial isolation, culture techniques, and identification methods. The systematic approach to bacterial characterization using morphological, biochemical, and antimicrobial testing yielded accurate identification of Staphylococcus aureus and Escherichia coli. The experiment reinforces the importance of proper laboratory technique, careful observation, and accurate documentation in microbiological analysis. These skills are essential for clinical microbiology, public health, and research applications.

References

• Tortora, G.J., Funke, B.R., & Case, C.L. (2019). Microbiology: An Introduction (13th ed.). Pearson.
• Clinical and Laboratory Standards Institute (CLSI). (2020). Performance Standards for Antimicrobial Susceptibility Testing. M100-ED30.
• Cappuccino, J.G., & Sherman, N. (2020). Microbiology: A Laboratory Manual (12th ed.). Pearson.
• Murray, P.R., Rosenthal, K.S., & Pfaller, M.A. (2021). Medical Microbiology (9th ed.). Elsevier.

Results & Grade

Final Grade: A+ (98/100)

The instructor commended the thorough documentation, accurate interpretation of biochemical test results, and professional presentation of data. The lab report demonstrated strong mastery of microbiology laboratory techniques, proper application of scientific method, and clear articulation of conclusions supported by empirical evidence.

Why This Lab Report Succeeded

• Adherence to aseptic technique and accurate culture handling
• Comprehensive documentation of morphological and biochemical data
• Clear tables summarizing antimicrobial susceptibility results
• Insightful discussion connecting laboratory findings to clinical relevance
• Accurate interpretation of Gram stain and biochemical test outcomes
• Proper citation of peer-reviewed microbiology sources
• Professional formatting consistent with scientific lab report standards