2. An Important Member of Microorganism Family—Bacteria

Microorganisms have such a large family, where its number of members significantly exceeds the total population of human beings on earth. Human infants lives in a sterile environment within the mother's womb before being born, but they interact with and get inhabited by various kinds of bacteria after being born. Apart from their skin, which is directly exposed to the air, a surprising number of bacteria also inhabit in our oral cavity, upper respiratory tract, intestine and the urogenital tract.

Let's take the oral cavity bacteria for example. How many bacteria live in a person's mouth? According to researchers, the number of bacteria existing in a well-cleaned oral cavity ranges from 1000 to 100,000; for someone who does not pay attention to cleaning his teeth, the number of bacteria on their teeth can reach up to a billion.

Interestingly, even though they all inhabit within the oral cavity, bacteria in your mouth vary greatly in terms of their type and species. They have their own living spaces insides your mouth. Some prefer to inhabit between the cheeks, some like to gather on the ventral surface of the tongue, others enjoy living on the back of the tongue. Anaerobic bacteria particularly like living between tooth gaps. These tiny creatures make up the large and magnificent kingdom of bacteria.

Bacteria are the most important members in the microorganism family, and they can be classified in several ways. Bacteria can be divided into two major groups by Gram stain: Gram-positive bacteria (colored purple) with thick cell walls and Gram-negative bacteria (colored red) with thin cellwalls. Additionally, they can also be distinguished by their shapes, such as cocci, bacillus and spirillum, under a microscope. The bacteria family has a family tree! Sicnetists use bacteria's genealogy to classify them according to their relatedness. Starting from: Kingdom, Phylum, Class, Order, Family, Genus and Species. Bacteria belong to the same species share a closer relationship.

Other than common structures such as cell membrane, cell wall, cytoplasm and the nuclear region, bacteria also have special appendages which enable them to move, grow and survive. We will talk about the special characteristics of bacteria below.

Pili

Many Gram-negative bacilli and a few Gram-positive cocci have short, hard and straight hair-like figures called pili. The diameter of a pilus is about 3～7 nm, and the length is about 0.5～6 μm, but some can grow up to 20 μm.

There are many types of pili, and scientists divided them into two major categories according to their functions—common pilus and sex pilus. Common pilus can help bacteria adhere to other cells firmly (including the human epithelial cells of the respiratory tract, digestive tract and urinary tract) or adhere to the surface of the object. Some can assist the bacteria to adhere to the erythrocyte, causing agglutination of erythrocyte. Sex pilus plays an extremely important role in the process of transferring genetic material. For example, if one bacterium wants to gift the resistance genes and virulence genes to another, sex pilus will act as a pipeline to transfer useful information between the two bacteria (Figure 1.5).

Flagella

Some bacteria have long, curving hair-like structures, called the flagella (Figure1.6), flagellum is a sporting organ of the bacteria. Flagella are longer than pili. In most cases, bacteria with flagella are vibrios, bacilli or individual cocci. According to their location, the flagella can be divided into three types: peri-flagella, lateral-flagella, and polar-flagella.Flagella move in through three ways:swimming in liquid, gliding on a solid surface, or rotating and shuttling in liquid. Bacteria with flagella are masters in sports!

Spores

Some bacteria are extremely clever, when they feel like their environment does not have enough nutrients to support their growth, they can form spores (Figure 1.7). Spores are like giant warehouses, allowing the bacteria to shift into dormacy like other animals. Spores are very strong, they are able to resist against heat, ultraviolet light, dryness, ionizing radiation, and many types of toxic chemicals.

For example, spores of botulinum can only be killed after 5～9.5 hours in boiling water. The spores' dormant ability is outstanding. Generally speaking, they can sleep and live from years to tens of years. According to record, some spore cells can sleep for hundreds to thousands of years. The most extreme case is found in the United States where a piece of amber contained spores that have been alive for 25 to 40 millions of years, you are still able to extract living spores from the bee's intestines. As soon as the nutrients within the environment are sufficient for the spore's growth, it can come back to life immediately. This is amazing!

Capsule

Some bacteria have a layer of loose myxoid structure on its surface (Figure 1.8), mainly composed of glucose and glucuronic acid, some are composed of polypeptide and lipid as well. The most important functions of capsule are anti-phagocytosis and the adhesion effects, which make the bacteria adhere on the surface of cells to avoid being killed by others. Gram-negative bacillus and Gram-positive coccus can produce capsule, such as Streptococcus pneumoniae and Klebsiella pneumoniae and etc.

What are the typical bacteria living within the environment, human body, and animals? Let's take a closer look.

2.1 Angels and Devils of Gram-positive Cocci

The nomination of Staphylococcus in Chinese is very interesting, as the morphology of this organism looks like clusters of grapes under the microscope(Figure 1.9). Most of them do not cause disease in human and animals.For example,Staphylococcus epidermidis is normal flora on the skin and plays a role as natural barriers for our skin.

The growth of Staphylococcus epidermidis in the other locations than the skin will cause disease in human. It will cause septicemia and bacterial endocarditis.Staphylococcus aureus is the most virulent species among Staphylococcus, which could cause a variety of terrible disease, including skin purulent infections, pneumonia, septicemia, food poisoning, shock and et al.

Streptococci are cocci that are grouped together like chains (Figure 1.10). Most of the Streptococci are normal groups of floras, and they are widely distributed out into the environment, and inside human and animal nasal area and their feces. For example, green streptococcus often inhabits within the oral cavity, respiratory tract, and digestive tract. Streptococcus pyogenes and Staphylococcus pneumoniae are the“devils” of Streptococcus, the former can cause skin and tonsil pyogenic infections, and it is also the cause of scarlatina; while the latter often causes meningitis, bronchitis, pneumonia and other diseases. These diseases can potentially be fatal.

Enterococci are a kind of round or oval shaped Gram-positive cocci that often occur as either single, in pairs, or short chains (Figure 1.11). They do not have spores and flagella. Most Enterococci need oxygen to survive, and they will die in an environment without oxygen.Enterococci are the normal flora of the intestinal tract which can help the body absorb nutrients, but under certain conditions, Enterococci may cause infections. Enterococcus faecalis and Enterococcus faecium being the most common causes of infection, they can lead to serious cases of cystitis, nephritis and septicemia.

2.2 Gram-negative bacilli

Gram-negative bacilli are shaped like rhabdoids (Figure 1.12), and they mainly exist in the intestine of human and animals. Because of the intestine's abundant nutrition and its optimum temperature, many bacteria enjoy settling in the intestine. The intestine is like a natural green house, nurturing various types of bacteria.

The Escherichia coli is one of the most commonly found examples. They arecommon florasliving in the intestine, and they play an important role in maintaining the balance within the intestine. However, when food and water are polluted and the number of Escherichia coli increases, the intestinal balance may be broken. In this situation, some subversives invade the intestine and cause gastroenteritis, therefore the count of Escherichia coli is used as a pollution index for food and drinking water.There are also a few bacteria that are completely fatal, such as Shigella and Salmonella, which cause bacillary dysentery and gastroenteritis. They are life threatening when serious conditions are reached.

Aside from Escherichia coli, other bacteria such as Klebsiella pneumoniae, Proteus Mirabilis, Pseudomonas Aeruginosa and Acinetobacter Baumannii are also important members of Gram- negative bacilli.

2.3 Anaerobic Bacteria

Some bacteria are named anaerobic bacteria by scientists because of their aversion to oxygen.Anaerobic bacteria living in human's intestine, such as Bi-fidobacterium and Lactobacillus, can synthesize various kinds of necessary vitamins for our growth and development. They can also take advantage of the protein residues to synthesize essential amino acid, and take part in the metabolism of sugar and protein. Additionally, they can promote the absorption of iron, magnesium, zinc and many other mineral elements. These nutrients are of great importance to our health, people can easily become sick without them.

However, some anaerobic bacteria are harmful to humans, such as Clostridium tetani,which are causes to tetanus. They are shaped like drumsticks, and they can live in the dry soil and dust for several years with plenty of vitality. Although Clostridium tetani are small size, they can easily take the life of a human. When people get wounded,Clostridium tetani are often found hinding in the deepest wounds, silently growing while releasing toxins to the body, which can make people go into convulsion, and eventually suffocate or die from respiratory failure. The Clostridium difficile is another example of a troublemaker, which is one of the main pathogens that cause diarrhea.

2.4 Zoonotic Pathogens

Some bacteria prefer the parasitic life on animals, and they make people sick through interactions with sick animals, causing zoonotic diseases. The most horrible bacteria are Yersinia pestis, which parasitize in rats and mice and cause extreme diseases such as the plague. After the rat dies, rat fleas will carry and spread the dangerous pathogens, Yersinia pestis, to humans by biting them. These horrible bugs can cause pneumonic plague, where the patients often die for coughing, chest pain, and dyspnea. The plague is also known as“black death”because the patient's skin will appear black after death.

In addition, they can also cause sepsis. The world-wide plague pandemic had occurred three times throughout history, and they have always triggered social turbulence. Eliminating rats can prevent the occurrence of the plague. After the establishment of the People's Republic of China, the movement of eliminating the four pests has been effective, and the plague have been effectively controlled.

Bacteria of all shapes and sizes fill our everyday lives and the environment we live in, they play an important role in the life activities of humans, animals and plants, and even for the circulation of materials on earth. As our understanding of the bacterial kingdom broadens, more secrets will unveil, and an increasing number of bacteria are studied and transformed into knowledge that will benefit mankind.