good response of the bone marrow. The primary bacteria-destroying cells (known as phagocytes) are the neutrophils, and the bone marrow should supply large numbers of these to combat the infection. The greater the “shift to the left” (increase in immature neutrophils), the more severe the infection. The appearance of numerous juvenile cells (metamyelocytes) indicates irritation of the bone marrow with regeneration. If the infection continues and the patient’s resistance declines, the shift advances further to the left. If improvement ensues, the shift declines and recedes to normal.  A falling white cell count with the number and maturity of neutrophils progressing toward normal indicates recovery.  A continued “shift to the left” with a falling total white cell count indicates a breakdown of the body’s defense mechanism and is a poor prognosis.  The percentage of eosinophils, lymphocytes, and monocytes generally decreases in acute infections.  In tuberculosis, an increase in monocytes (monocytosis) indicates activity in the infected area. An increase in lymphocytes (lympho- cytosis) indicates healing.  Eosinophils increase in parasitic infections and allergic conditions. BACTERIOLOGY LEARNING OBJECTIVE: Recall bacteria classifications, common bacteria, and procedural steps for making smears, Gram staining, and reading and reporting smears. Bacteriology is the study of bacteria. Of primary interest to Hospital Corpsman is medical bacteriology, which deals with the bacteria that cause disease in man. Bacteria are prokaryotic microorganisms of the kingdom Protista. They reproduce asexually by transverse binary fission in which the cell divides into two new cells. Bacteria are found almost everywhere, and the human body harbors vast numbers. Many bacteria are beneficial and essential to human life; only a few are harmful to man. BACTERIA CLASSIFICATION Since there are thousands of types of bacteria, a method of classification is essential. Bacteria are classified according to their respective  disease-producing ability,  growth requirements,  morphologic characteristics,  colonial morphology,  toxins produced, and  Gram’s stain reaction. Disease-Producing Ability The disease-producing ability of bacteria is referred to as either pathogenic or nonpathogenic. Pathogens are bacteria that cause diseases, and nonpathogens are harmless bacteria. Bacteria that are essential to our body are, in their proper environment, called common or normal flora. For example, alpha streptococcus in the throat is common flora, but when it is found elsewhere (such as in the blood stream, possibly as a result of tooth extraction), it may cause diseases such as septicemia and endocarditis. Growth Requirements The four growth requirements for bacteria are  temperature,  oxygen,  nutrition, and  moisture. TEMPERATURE REQUIREMENTS . — Temperature requirements are divided into the following three categories.  Psychrophilic—bacteria that reproduce best at 15EC to 20EC  Mesophilic—bacteria that reproduce best at 20EC to 45EC  Thermophilic—bacteria that reproduce best at 50EC to 55EC OXYGEN REQUIREMENTS.—The amount of oxygen needed for an organism to grow or reproduce varies with the type of organism. Aerobes are organisms that reproduce in the presence of oxygen. Obligate aerobes are organisms that grow only in the 7-25