In some bacterial species (notably the Gram positive genera Bacillus and Clostridium), endospores may be formed as the culture nears death phase, these are tough dry spherical bodies that can endure long periods (often many years) in harsh chemical and climatic conditions, and will germinate to produce a new bacterial cell when conditions are suitable. Special microscope slide counting chambers are used to count cells in fluid cultures, and there are a number of automated instruments that will do the same thing. Or, a process called serial dilution and plating onto agar is used and I will describe this in class or tutorial. How quickly a bacterial culture increases in numbers depends on a variety of factors, such as temperature, pH, oxygen content, amount and type of nutrient. There is an optimum value for all of these factors for the best growth of any given bacterium. Many bacteria are inhibited from growing at acid pH’s, but there is an important group of bacteria – the acidophiles, of numerous different species, that prefer to grow at acid pH (generally down around pH 2-4), indeed many of them actually produce acids, and some acidophiles are important in food production, such as those involved in yogurt production. Temperature: Psychrophiles grow best at lower but not excessively colder temperatures (15-20 C but sometimes as low as 0 C, a few pathogens are psychrophiles), mesophiles at 25-40 C (thus most bacteria that are human pathogens are mesophiles), thermophiles at >40 C. Some bacteria (like some that associate with deep sea volcanic vents) have to have high temperature in order to grow, sometimes up around the boiling point of water, these are obligate thermophiles, some of these are economically important (one of them provides an essential enzyme used in DNA amplification for criminal DNA fingerprinting). It is obvious that most human pathogenic bacteria are mesophiles, since this is within the temperature range of the human body (approximately 37 C). Oxygen. Some bacteria will not grow unless oxygen is present, these are obligate aerobes. Some bacteria need oxygen to grow, but at very low levels, these are microaerophiles, some bacteria must have NO oxygen present in order to grow, these are obligate anaerobes, in fact some obligate anaerobes, such as the Clostridia, are actually killed by exposure to even small amounts of oxygen.