Bacteria Growth - Unit 1 Lesson 3 Read TEXT PDF

What do bacteria need to live and grow? What do bacteria need to live? Like all organisms, bacteria require food, water, and space to live. When biologists (and biology students) catch and grow bacteria, they must provide these requirements for the bacteria. Some bacteria are able to make their own...

What do bacteria need to live and grow? What do bacteria need to live? Like all organisms, bacteria require food, water, and space to live. When biologists (and biology students) catch and grow bacteria, they must provide these requirements for the bacteria. Some bacteria are able to make their own food through a process of photosynthesis, but others obtain food from their environments. Biologists can grow bacteria in liquid mediums that contain food molecules the bacteria take up and use to make the molecules they need. Another common way to grow bacteria is on a firm medium on a petri plate. In this case, agar is added to the liquid medium. Agar is a substance that is liquid at high temperatures, but becomes solid at lower temperatures, similar to gelatin. While the liquid containing agar is still hot, it is poured into petri plates. As it cools, it sets and forms a solid surface where bacteria can grow. When bacteria are transferred to this medium, they use the nutrients and water there to grow. What happens when bacteria grow? What does “bacteria growth” mean? Does it mean an individual bacterium gets bigger and bigger? There is a limit to how big an individual bacterium can grow, so bacteria growth almost always means an increase in the number of bacteria in a bacteria population. Each individual spot on the petri plate in the figure above is a colony of bacteria. It shows the spot where a single bacterium landed and began growing and dividing. In other words, the bacterium began using the food and water in the medium to reproduce itself. By the time a colony is large enough to be seen on a plate, there are a million or more bacteria growing there. When a bacterium reaches its maximum size, it copies its genetic material (DNA) and splits in two. The two new bacteria are exact copies of the original bacterium. This is an example of asexual reproduction. (In asexual reproduction there is only one parent, so the offspring do not receive a combination of genetic information from two different parents.) How fast do bacteria grow? Compared to most organisms, bacteria can grow very quickly. If they have enough food, water, and space, some bacteria can reproduce every 20 to 30 minutes. Imagine starting with one bacterium on a petri plate. In 30 minutes, there would be two bacteria on the plate. After another 30 minutes, each of those bacteria would divide and there would be four bacteria. Assuming this growth rate continues, how many bacteria would there be after 6 hours? How long would it take to reach 1 million bacteria? The table below shows a mathematical model that answers these questions. The number of bacteria in the population doubles in each 30-minute generation. What observations can you make about this table? Generation Time (min/hours) Population size 0 0/0 1 1 30/0.5 2 2 60/1.0 4 3 90/1.5 8 4 120/2.0 16 5 150/2.5 32 6 180/3.0 64 7 210/3.5 128 8 240/4.0 256 9 270/4.5 512 10 300/5.0 1024 11 330/5.5 2048 12 360/6.0 4096 13 390/6.5 8192 14 420/7.0 16,384 15 450/7.5 32,768 16 480/8.0 65,536 17 510/8.5 131,072 18 540/9.0 262,144 19 570/9.5 524,288 20 600/10.0 1,048,576 21 630/10.5 2,097,152 Number of bacteria in a population at various times, growing under optimal conditions. Optimal conditions means that there is enough food, water, and space for the bacteria at all times. Bacteria growth under optimal conditions. Starting with one bacterium, there will be fewer than 5,000 bacteria after generation 12 at 6 hours. The population appears to be growing slowly at that point. But just 4 hours later in generation 20, the population is over 1 million. A half hour later there are over 2 million bacteria. The population grows slowly at first and then very quickly. The J-shaped curve in the graph shows exponential growth. A simple way to think about exponential growth is that the larger a population gets, the faster it grows. Populations of all species, from fast-growing bacteria to slow-growing humans, have the potential to grow exponentially, with one big assumption. That assumption is that there is always enough food, water, and space available to support the growing population. In nature this is rarely true. Bacteria growing on petri plates or in flasks with liquid medium have much better growth conditions than bacteria in nature. But even in the laboratory, they eventually run out of food and space. Bacteria begin to die as they compete for food, and population growth slows down. Revised Growth Model. Growth curve for bacteria in nature. The figure above shows a typical growth curve for organisms grown under natural conditions. Just like exponential growth, the population grows slowly at first and then very quickly. But instead of continuing to grow, the population size stays about the same after it reaches a certain level. At this point, food and space begin to limit the number of individuals that can be supported. The number of organisms added to the population and the number dying are about the same, so the population size remains constant. What factors affect the rate of bacteria growth? If they have enough food, water, and space, bacteria will continue to grow. But they grow more slowly under some conditions than others. One factor that affects the rate of bacteria growth is temperature. Bacteria that live on and in humans grow fastest at the warm temperatures maintained by the body. On the other hand, bacteria that live in soil and lakes grow faster at cooler temperatures. Some bacteria live in hot springs or compost piles (which can get very hot). They grow best at high temperatures that would kill many bacteria. The acidity (pH) and salinity (saltiness) of the environment are two other factors that affect bacterial growth. Many bacteria grow best at neutral pH, but some thrive in acidic or basic environments. Many bacteria cannot tolerate high salt concentrations. Before refrigeration was available, people used salt-curing to preserve meat. The high concentrations of salt killed most unwanted bacteria. Some bacteria, however, can tolerate high salt concentrations.