Streaking is a technique used to isolate a pure strain from a single microorganism. Micro 201L, 302, 302L, 310L, 440

Sometimes a solution of bacteria is too concentrated to work with. A serial dilution is a series of successive, measured dilutions in order to obtain a known, workable concentration of bacteria.

Because bacteria are, for the most part, transparent, we use stains to give them color for microscopic observation. Making a bacterial smear prepares the bacteria to be stained and a simple stain is a quick and easy way to observe bacteria.

The spread plate method is a technique to plate a liquid sample containing bacteria so that the bacteria are easy to count and isolate. A successful spread plate will have a countable number of isolated bacterial colonies evenly distributed on the plate. 

The Gram stain is a fundamental tool used to differentiate bacteria by placing them into one of two general groups. Bacteria that are a deep purple after staining are called Gram + (positive), while bacteria that appear a lighter pink are called Gram - (negative.) Becoming familiar with the Gram stain technique is essential and it is often the first step in identifying an unknown bacterium. 

Some bacteria produce endospores in response to stressful environmental conditions. These endospores are dormant cells that are able to survive until conditions improve, then become active and proliferate once they do. The endospore stain causes these unique structures to be easily visible and can help determine if they are present in a particular culture.

The carbohydrate fermentation test is used to determine whether or not a bacteria can utilize a certain carbohydrate. It tests for the presence of acid and/or gas produced from the fermentation of a single particular carbohydrate.

The catalase test tests for the presence of catalase, an enzyme that breaks down the harmful substance hydrogen peroxide into water and oxygen. If an organism can produce catalase, it will produce bubbles of oxygen when hydrogen peroxide is added to it.

The oxidase test determines whether or not a bacterial isolate produces the enzyme cytochrome c oxidase, which is an important part of the electron transport chain.

Some bacteria are able to break down blood cells by a process called hemolysis. Knowing what type of hemolysis a bacterial strain is capable of can be helpful in identifying several types of bacteria, especially organisms isolated from human tissue such as Streptococcus and Staphylococcus species.

The IMViC series is a group of four individual tests that are commonly used to identify bacterial species, especially coliforms. The capital letters in ‘IMViC’ each stand for one of the four tests: I for Indole test, M for Methyl Red test, V for Voges-Proskauer test, and C for Citrate test.

The nitrate reduction test tests for a bacterial strain’s ability to reduce nitrate to nitrite or further compounds.

Triple sugar iron agar, or TSI, is a differential medium that tests a bacterial strain for several different properties at once. It tests for acid and gas production from the fermentation of glucose and sucrose and/or lactose and for the production of hydrogen sulfide.

Measuring water quality is an important application of microbiology. For example, residential or agricultural sewage runoff could contaminate a source of water and make it unsafe to drink. One measure of water quality is the prevalence of coliform bacteria in the water. Coliforms are easy to identify and their presence in high numbers could indicate dangerous levels of fecal or other contamination, which is why they are important indicator organisms.

Members of the American Society for Microbiology share their stories of how they discovered microbiology.