Category Archives: Microbiology for Beginners

Catalase test: principle, uses, procedure and results

catase test

Catalase Test: Positive and Negative results

Catalase is an enzyme, which is produced by microorganisms that live in oxygenated environments to neutralize toxic forms of oxygen metabolites; H2O2.  The catalase enzyme neutralizes the bactericidal effects of hydrogen peroxide and protects them. Anaerobes generally lack the catalase enzyme.

Catalase mediates the breakdown of hydrogen peroxide H2O2 into oxygen and water.  To find out if a particular bacterial isolate is able to produce catalase enzyme, small inoculums of bacterial isolate is mixed into hydrogen peroxide solution (3%) and the rapid elaboration of oxygen bubbles occurs.  The lack of catalase is evident by a lack of or weak bubble production.

Seven common characteristics of family Enterobacteriaceae

Enterobacteriaceae family contains a large number of genera that are biochemically and genetically related to one another. Many of the traditional or familiar bacteria are found in this family e.g. Escherichia, Shigella, Salmonella, Enterobacter, Proteus, Yersinia etc.

Seven common characteristics of family Enterobacteriaceae are:

  1. They are gram negative, short rods
  2. They are non-sporulating, facultative anaerobes
  3. These organism have simple nutritional requirements and MacConkey agar is used to isolate and differentiate  organisms of Enterobacteriaceae family (Pink colored colonies of lactose fermenter-coliforms and pale colored colonies of Non lactose fermenter)
    LF and NLF colonies in MacConkey Agar

    LF and NLF colonies in MacConkey Agar

    Lactose fermenters: (CEEK)

    1. Citrobacter
    2. Escherichia
    3. Enterobacter
    4. Klebsiella

    Non lactose fermenter (ShYPS)

    1. Shigella
    2. Yersinia
    3. Proteus
    4. Salmonella

What are the roles of Medical Microbiologist?

Microbiologists work in public health laboratories, hospital laboratories, reference or independent laboratories, and physician office laboratories.

Microbiologist must be aware of what the physician needs and as well as what the laboratory needs.

At an elementary level, the physician needs answers to three basic questions from the microbiology laboratory:


Role of Microbiologist: Cartoon by Czichos

  1. Is my patient’s illness caused by a microbe?
  2. If so, what is it?
  3. What is the antibiotic susceptibility profile of that organism so that therapy can be targeted?

General and differential characteristics of Gram positive and Gram Negative bacteria

The cell wall of Gram negative bacteria is more complex than those of Gram Positive bacteria. Gram negative bacteria contain an extra layer of cells called outer membrane or LPS layer which surrounds the thin peptidoglycan layer. LPS layer is absent in Gram positive bacteria.

Difference between Gram positive and Gram Negative bacterial cell wall

Difference between Gram positive and Gram Negative bacterial cell wall

Some of the main differences between Gram Positive bacteria and Gram Negative bacteria are:

Types of Staining techniques used in Microbiology and their applications

Acid fast stain (Ziehl-Neelsen technique):  It distinguishes acid fast bacteria such as Mycobacterium spp from non-acid fast bacteria; which do not stain well by the Gram Staining.  It is used to stain Mycobacterium species  (Mycobacterium tuberculosis, M. ulcerans and M. leprae)

Acridine Orange Stain: This staining method is used to confirm the presence of bacteria  in blood cultures when Gram stain  results are difficult to interpret or when presence of bacteria is highly suspected but none are detected using light microscopy. Acridine orange binds to nucleic acid and stains them. It is also used for the detection of Mycoplasmas (cell wall deficient bacteria)

Auramine-Rhodamine technique: This fluorochrome staining  method is used to enhance the detection of mycobacteria directly in patient specimens and initial characterization of cells grown in culture.