What Are Microbes?
Microbes are tiny organisms -too tiny to see without a microscope, yet they are abundant on Earth. They live everywhere —in air, soil, rock, and water. Some live happily in searing heat,while others thrive in freezing cold.Some microbes need oxygen to live,but others do not. These microscopic organisms are found in plants and animals as well as in the human body. Some microbes cause disease in humans, plants, and animals. Others are essential for a healthy life, and we could not exist without them.Indeed,the relationship between microbes and humans is delicate and complex.
Most microbes belong to one of four major groups: bacteria,viruses,fungi,or protozoa. A common word for microbes that cause disease is “germs.”
Since the 19th century,we have known microbes cause infectious diseases .Near the end of the 20th century, researchers began to learn that microbes also contribute to many chronic diseases and conditions.Mounting scientific evidence strongly links microbes to some forms of cancer, coronary artery disease,diabetes,multiple sclerosis, and chronic lung diseases.
Microbes belonging to the bacteria group are made up of only one cell .Under a microscope,bacteria look like balls, rods,or spirals.Bacteria are so small that a line of 1,000 could fit across the eraser of a pencil.Life in any form on Earth could not exist without these tiny cells. Scientists have discovered fossilized remains of bacteria that date back more than 3.5 billion years,placing them among the oldest living things on Earth.Bacteria can inhabit a variety of environments,including extremely hot and cold areas.
Psychrophiles,or cold-loving bacteria,can live in the subfreezing temperature of the Arctic. Thermophiles are heat-loving bacteria that can live in extreme heat.
Extreme thermophiles,or hyperthermophiles,thrive at 235 degrees Fahrenheit near volcanic vents on the ocean floor.
Many bacteria prefer the milder temperature of the healthy human body.
Like humans,some bacteria (aerobic bacteria)need oxygen to survive.Others (anaerobic
bacteria),however,do not. Amazingly,some can adapt to new environments by learning
to survive with or without oxygen.
Like all living cells,each bacterium requires food for energy and building materials.There are countless numbers of bacteria on Earth —most are harmless and many are even beneficial to humans.In fact,less than 1 percent of bacteria cause diseases in humans.For
example,harmless anaerobic bacteria,such as Lactobacilli acidophilus ,live in our intestines,where they help to digest food,destroy disease-causing microbes,fight cancer cells,and give the body needed vitamins.Healthy food products,such as yogurt,sauerkraut,and cheese, are made using bacteria.
Some bacteria produce poisons called toxins ,which also can make us sick.
Viruses are among the smallest microbes,much smaller even than bacteria.Viruses are not cells.They consist of one or more molecules of DNA or RNA ,which contain the virus ’s genes surrounded by a protein coat.Viruses can be rod-shaped,sphere-shaped,or
multisided.Some viruses look like tadpoles.
Unlike most bacteria,most viruses do cause disease because they invade living,normal cells,such as those in your body.They then multiply and produce other viruses
like themselves.Each virus is very particular about which cell it attacks.
Various human viruses specifically attack particular cells in your body ’s organs,systems,or tissues ,such as the liver,respiratory system,or blood. Although types of viruses behave differently,most survive by taking over the machinery that makes a cell work.
Briefly,when a piece of a virus,called a virion,comes in contact with a cell it likes,it may attach to special landing sites on the surface of that cell.From there,the virus may inject molecules into the cell,or the cell may swallow the virion.Once inside the cell,viral molecules such as DNA or RNA direct the cell to make new virus offspring.That ’s
how a virus infects a cell.
Viruses can even “infect ” bacteria..These viruses,called bacteriophages,may help researchers develop alternatives to antibiotic medicines for preventing and treating
Other viral infections can result in deadly diseases such as AIDS (acquired immunodeficiency syndrome)or Ebola hemorrhagic fever.
A fungus is actually a primitive plant.Fungi can be found in air,in soil,on plants,and in water.Thousands,perhaps millions,of different types of fungi exist on Earth.The most familiar ones to us are mushrooms,yeast,mold,and mildew.Some live in the human body,usually without causing illness.Fungal diseases are called mycoses.
Mycoses can affect your skin,nails,body hair, internal organs such as your lungs,and body
systems such as your nervous system.Aspergillus fumigatus , for example,can cause aspergillosis,a fungal infection in your respiratory system.
Some fungi have made our lives easier.Penicillin and other antibiotics,which kill harmful bacteria in our bodies,are made from fungi.Other fungi,such as certain yeasts,also can be helpful.For example,when a warm liquid,such as water,and a food source are added to
certain yeasts,the fungus ferments.The process of fermentation is essential for making healthy foods like some breads and cheeses.
Protozoa are a group of microscopic one-celled animals. Protozoa can be parasites or predators.In humans, protozoa usually cause disease.
MICROBES IN THE HEALTHY HUMAN BODY*
Microbes found in
| Ear (outer)
|| Aspergillus (fungus)
|| Candida (fungus)
| Small intestine
|| Escherichia coli
|| Gardnerella vaginalis
|| Staphylococcus aureus
|| Streptococcus salivarius
| Large intestine
|| Trichomonas hominis (protozoa)
*A selection of usually harmless microbes,some of which help keep our bodies functioning normally.If their numbers become unbalanced,however,these microbes may make us sick.All are bacteria,unless otherwise noted.
Some protozoa,like plankton,live in water environments and serve as food for marine animals,such as some kinds of whales.Protozoa also can be found on land in decaying matter and in soil,but they must have a moist environment to survive.Termites wouldn ’t be able to do such a good job of digesting wood without these microorganisms in their guts.
Malaria is caused by a protozoan parasite.Another protozoan parasite,Toxoplasma gondii ,causes toxoplasmosis in humans.This is an especially troublesome infection in pregnant women because of its effects on the fetus,and in people with HIV infection or other immune deficiency disorder.