The world of bacteria
Bacteria are tiny organisms studied by microbiology, visible only under a microscope. In the laboratory they are grown in sterile conditions on a nutrient medium in a Petri dish; they multiply and form a colony that can be seen with the eye. By outer shape they are cocci (round), bacilli (rod-shaped), and vibrios and spirilla (curved). Bacteria have no nucleus, so they are prokaryotes. In good conditions some bacteria divide in two every 20–30 minutes. In bad conditions they form a thick coat and become a spore that can survive a long time. Useful bacteria include fermenting bacteria (used for yoghurt and pickles), decomposing bacteria (which break down dead remains and act as nature’s “sanitary workers”) and nodule bacteria (living in the roots of legumes and passing atmospheric nitrogen to the plant – a symbiosis that benefits both). Parasitic bacteria cause diseases such as tuberculosis, cholera and tetanus. The mass spread of a disease is an epidemic; to prevent it a quarantine is declared. The simplest protection is washing hands with soap before eating. Plants such as juniper and poplar release phytoncides that clean the air of harmful bacteria.
Hygiene exercise: the teacher shows the steps of washing hands properly with soap for 20 seconds (palms, backs, between fingers, nails) and pupils repeat. Then, in pairs, draw a map “Where do bacteria work?”: the kitchen (yoghurt, pickles), the field (nodule bacteria), the forest (decomposers), the hospital (disease-causing bacteria and protection from them). Safety: never open or sniff Petri dishes with grown bacteria – only the teacher handles them.