MCB2010L Microbiology Lab Quiz 2 Study Guide PDF

MCB2010L Microbiology Lab Quiz 2 Study Guide Bacterial Size, Shape, and Arrangement Bacteria may be spherical coccus (plural: cocci), rod-shaped bacillus (plural: bacilli), and spiral. Cocci that divide and remain attached in chains are called streptococci. Those that divide and remain attached in...

MCB2010L Microbiology Lab Quiz 2 Study Guide Bacterial Size, Shape, and Arrangement Bacteria may be spherical coccus (plural: cocci), rod-shaped bacillus (plural: bacilli), and spiral. Cocci that divide and remain attached in chains are called streptococci. Those that divide and remain attached in grapelike clusters or broad sheets are called staphylococci. Spiral bacteria have one or more twists. They are never straight. Microbes and Oxygen Obligate Aerobes – Only aerobic growth. Oxygen is required for survival. Growth occurs only where high concentrations of oxygen have diffused into the medium, Presence of enzymes catalase and superoxide dismutase (SOD) allows toxic forms of oxygen to be neutralized. Facultative Anaerobes – Both aerobic and anaerobic growth although greatest growth occurs in the presence of oxygen. Growth is best where most oxygen is present, but occurs throughout tube. Presence of enzymes catalase and superoxide dismutase (SOD) allows toxic forms of oxygen to be neutralized. Escherichia coli is an example of a facultative anaerobe. Obligate Anaerobes – Only anaerobic growth. Growth ceases in presence of oxygen, and most are harmed by it. Growth occurs only where there is no oxygen. Lacks the enzymes to neutralize the toxic forms of oxygen. Clostridium species are obligate anaerobes. Aerotolerant Anaerobes – Only anaerobic growth, but growth continues in the presence of oxygen. Growth occurs evenly in test tube because oxygen has no effect. Presence of enzymes catalase and superoxide dismutase (SOD) allows toxic forms of oxygen to be neutralized. Microaerophiles – Only aerobic growth but oxygen required in low concentration. Growth occurs only where a low concentration of oxygen has diffused into the medium. Produce lethal amounts of toxic forms of oxygen if exposed to normal atmospheric oxygen. Culture Media Reducing media are designed to grown obligate anaerobic bacteria. These media contain ingredients, such as sodium thioglycolate, that chemically combine with dissolved oxygen and deplete the oxygen in the culture medium. Selective media are designed to suppress the growth of unwanted bacteria and encourage the growth of the desired microbes. Differential media make it easier to distinguish colonies of the desired organism from other colonies growing on the same plate. Gram Stain The cell wall of the bacterial cell is a complex, semirigid structure responsible for the shape of the cell. The major function of the cell wall is to prevent bacterial cells from rupturing when the water pressure inside the cell is greater than that outside the cell. The bacterial cell wall is composed of a macromolecular network called peptidoglycan. Peptidoglycan consists of a repeating disaccharide connected by polypeptides to form a lattice that surrounds and protects the entire cell. In most gram-positive bacteria, the cell wall consists of many layers of peptidoglycan. The cell walls of gram-negative bacteria consist of one or a very few layers of peptidoglycan and an outer membrane. Crystal violet, the primary stain, stains both gram-positive and gram-negative cells purple. Iodine, the mordant, forms large crystals with the dye in the cytoplasm, forming the crystal violet-iodine complex. Alcohol (decolorizer) dissolves the outer membrane of gram-negative cells, allowing the crystal violet-iodine complex to escape. Gram-negative cells are therefore colorless and no longer visible after the alcohol wash. The alcohol has no effect on gram-positive cells, so they retain the purple dye. Because gram-negative bacteria are colorless after the alcohol wash, the addition of safranin (the counterstain) turns the cells pink. Although both gram-positive and gram-negative cells both absorb safranin, the pink color of safranin is masked by the darker purple dye previously absorbed by gram-positive cells.