Fundamentally, there are three different types of bacteria; Gram-positive, Gram-negative, and those bacteria that are Gram-indeterminate. Gram staining is a technique that involves a primary stain, mordant, decolorizer, and secondary stain. Once applied to a pure bacterial smear on a microscope slide, the scientist can see the cellular shape of the bacterium as well as the Gram-reaction (positive, negative or indeterminate). Gram reactions are based on the outer cell wall structure of the applicable bacteria and either appear purple (Gram positive) or pink (Gram negative) under 100X (with 10X eyepiece) oil immersion microscopy. Once the bacterium is stained, it allows the scientist to note the cellular shape of the organism.
Bacteria are classified by shape as either coccus or rod-shaped. Coccus bacteria can be subtyped into coccus, dicoccus, streptococcus etc. Rods or bacilli shaped bacteria are also subtyped as streptobacilli, bacilli, diplobacilli and palisades. There are also other cellular shapes like spirilla, vibrios, spirochetes, and Corynebacterium.
Gram staining is an inexpensive, effective, quick, and relatively easy method of analysis. It is a foundational step in identifying most of the contaminating bacteria found in food and pharmaceutical recalls. While the technique is over 150 years old, and is generally considered less informative than molecular identification, it is still commonly relied upon in microbiology labs worldwide. It is very effective for first response notification. Listeriawill always be Gram-positive and E.coli will always be Gram-negative. Therefore, if your compressed air sample report states that your air contains Gram-positive bacteria and you’re only concerned with coliforms, (Gram-negative bacteria found in stool and intestines of warm-blooded animals) then you won’t need to spend the extra money on molecular techniques for identification.