bacteria

Bacteria (singular bacterium) are a type of biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a number of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria inhabit soil, water, acidic hot springs, radioactive waste, and the deep biosphere of the earth's crust. Bacteria also live in symbiotic and parasitic relationships with plants and animals. Most bacteria have not been characterised, and only about 27 percent of the bacterial phyla have species that can be grown in the laboratory. The study of bacteria is known as bacteriology, a branch of microbiology.

细菌是一种生物细胞。它们构成了原核微生物的一个大域。通常只有几微米长，细菌有许多形状，从球形到杆状和螺旋状。细菌是最早出现在地球上的生命形式之一，并且存在于地球的大部分栖息地。细菌栖息在土壤、水、酸性温泉、放射性废物和地壳深层生物圈中。细菌还与动植物存在共生和寄生关系。大多数细菌还没有被描述，只有大约27%的细菌门有可以在实验室中生长的物种。对细菌的研究被称为细菌学，是微生物学的一个分支。

Nearly all animal life is dependent on bacteria for survival as only bacteria and some archaea possess the genes and enzymes necessary to synthesize vitamin B12, also known as cobalamin, and provide it through the food chain.

几乎所有动物的生存都依赖于细菌，因为只有细菌和一些古细菌拥有合成维生素B12（也称为钴胺素）并通过食物链提供所需的基因和酶。

There are typically 40 million bacterial cells in a gram of soil and a million bacterial cells in a millilitre of fresh water. There are approximately 5×10E30 bacteria on Earth, forming a biomass which is only exceeded by plants.

一克土壤中通常有4000万个细菌细胞，一毫升淡水中有100万个细菌细胞。地球上大约有5×10E30个细菌，形成了只有植物才能超过的生物量。

In humans and most animals, the largest number of bacteria exist in the gut, and a large number on the skin. The vast majority of the bacteria in the body are rendered harmless by the protective effects of the immune system, though many are beneficial, particularly in the gut flora. However, several species of bacteria are pathogenic and cause infectious diseases, including cholera, syphilis, anthrax, leprosy, and bubonic plague. The most common fatal bacterial diseases are respiratory infections.

在人类和大多数动物中，肠道中的细菌数量最多，皮肤上的细菌数量也最多。由于免疫系统的保护作用，人体内的绝大多数细菌变得无害，尽管许多细菌是有益的，尤其是在肠道菌群中。然而，有几种细菌是致病性的，会引起传染病，包括霍乱、梅毒、炭疽、麻风病和黑死病。最常见的致命细菌性疾病是呼吸道感染。

The two most common causative agents of infectious disease are the virus and bacterium. Both of these pathogens are invisible to the naked eye, allowing for their stealthy transfer from person to person during an outbreak of a contagious disease. While they rightly share a nasty reputation as disease agents, their properties apart from the harm they cause are quite dissimilar.

传染病最常见的两种病原体是病毒和细菌。这两种病原体都是肉眼看不见的，因此在传染病暴发期间，它们可以在人与人之间秘密转移。虽然它们作为疾病媒介有着令人厌恶的名声，但它们的性质除了造成的伤害外，却截然不同。

Viruses are not living organisms, bacteria are. Bacteria are giants when compared to viruses. Viruses infect a host cell and then multiply by the thousands, leaving the host cell and infecting other cells of the body. The infection caused by pathogenic bacteria is usually confined to a part of the body, described as a localized infection. These infections may be caused by the bacteria themselves or by toxins (endotoxins) they produce. Examples of bacterial disease include pneumonia, tuberculosis, tetanus, and food poisoning.

病毒不是活的有机体，细菌才是。与病毒相比，细菌是巨大的。病毒感染宿主细胞，然后成千地繁殖，离开宿主细胞，感染身体的其他细胞。病原菌引起的感染通常局限于身体的一部分，称为局部感染。这些感染可能是由细菌本身或它们产生的毒素（内毒素）引起的。细菌性疾病的例子包括肺炎、肺结核、破伤风和食物中毒。

Viruses can infect bacteria.