MCAT Biological and Biochemical Foundations of Living Systems: Passage 10 — Flashcards

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Compared with their eukaryotic counterparts, bacteria are small and have simple structures. Their genomes are a mere 1 to 5 million nucleotides, compared with the 12 million of yeast and 3 billion of humans. Despite their diminutive size, a subset of these organisms can cause devastating disease in humans, animals, and plants. When a bacterial infection is suspected in a clinical setting, the Gram stain is often the first test that is performed, which allows classification of the bacteria as either Gram-positive or Gram-negative. Gram-positive bacteria, such as Staphylococci, possess a single membrane and a cell wall that is thick and is composed of cross-linked peptidoglycan. Bacteria in this group retain the violet dye used in the Gram stain procedure. Gram-negative bacteria, such as Salmonella, possess two membranes that are separated from each other by a periplasmic space. The cell wall is located in this space in Gram-negative bacteria, and it is thinner than in Gram-positive bacteria; as a result, Gram-negative bacteria cannot retain the violet dye used in Gram staining.

Bacteria can be also be classified based on their shape: spheres (cocci), such as Streptococcus; rods (bacilli), such as E. coli; and spiral cells (spirochetes). However, many bacteria have similar shapes within the Gram-positive and Gram-negative categories. Another way to distinguish between different bacteria is based on the proteins on the cell surface. Many bacteria have helical flagella on their surface that they use for propulsion, or pili spikes that allow them to adhere to host tissue. There are also more specific tests. For example, the Lancefield system groups Streptococci (groups A–H, K–M, O–V) based on the carbohydrate groups on their surface, which can be detected using antibody-based tests. Some tests rely on enzymatic activity. The catalase test differentiates aerobic and anaerobic bacteria because only the former express high levels of the catalase enzyme, which converts hydrogen peroxide to hydrogen and oxygen. The presence of catalase can be detected in this test because the bacteria produce bubbles in the presence of hydrogen peroxide. Other enzyme tests involve growing the bacteria on specialized agar plates. For example, lactose-fermenting enteric bacteria, including E. coli and Klebsiella, can be identified on MacConkey agar. These bacteria ferment the lactose in the agar, causing the pH of the lactose to change and the agar to undergo an easily perceptible change in color. Staphylococcus aureus can be distinguished from other Staphylococci species because it has coagulase activity, which results in hemolysis of red blood cells on a blood agar plate. These and a panel of other tests are routinely performed in clinical microbiology labs, either by hand or automated machines, to identify organisms from patient samples.