Hello thanks for your question Bacteria are present every where, even on our body surface, even in our body, in our ears, nose etc. But the thing is there are good bacteria (beneficial or in better term commensals) or bad bacteria (harmful). The different bacterial species tend to produce different diseases, different symptomatology for example a bacteria Staphylococcus aureus is involved in food poisoning, many chronic infections. Pneumococcus is involved in pneumonia. They have different temperature range to grow, and most of bacteria grow at ambient room temperature with high humidity. Extremes of temperature whether lower or upper is harmful to bacteria. Most of bacteria which cause food poisoning usually grow more in rainy season or in summers. On the contrary commensels have many beneficial effects, they protect our body from infection by other bacteria. They help in digestion of food. food poisoning bacteria are usually not harbored in nose mouth or on skin of normal person. They are usually transmitted from one infectious person to food and then from food to person and so on. thanks hope i clarified your doubt to some extent take care