Microterrors: The Complete Guide to Bacterial, Viral and Fungal Infections That Threaten Our Health by Tony Hart

From the Publisher

The hidden dangers surrounding us.

Despite the confident strides of modern science, the threat of deadly unseen organisms such as viruses, bacteria and fungi still grip the imagination with their ferocious intensity.

For instance, resistant strains of bacteria can now survive the strongest antibiotics and deadly new biological weapons are being cooked up in laboratories worldwide.

Microterrors explores these threats as well as humanity's greatest living rivals that have been on the planet far longer than we have.

The introduction covers terms, definitions and a brief natural history, including the role of viruses in human evolution, as well as bioengineering and biological weapons. Microterrors then presents the truly terrifying rogues' gallery of invisible killers.

Dramatic life-like digital illustrations and computer-colored electron images provide mug book profiles of hundreds of naturally occurring and bioengineered microterrors, including:

• Ebola, bubonic plague, cholera, malaria


• Bacteria: pneumonia, anthrax, botulism


• Viruses: HIV, hepatitis, common cold


• Fungi and blood and tissue diseases.

Each entry includes concise facts such as date of discovery and place of origin, period of incubation, symptoms and length of suffering, likelihood of death, treatments, vaccines and cures.

• Table of Contents
• Forewords & Introductions

Table of Contents

Forewords & Introductions

Introduction: What are microbes?


They are invisible to us, but each day we are exposed to a multitude of different microorganisms. For the most part they do us no harm, but occasionally we are challenged by disease-causing (pathogenic) microbes that can be detrimental to our health.

All the microorganisms that live on us and in our environment are at the lowest segment of the trunk of the evolutionary tree. Yet what they lack in sophistication and superior development, they overcome by tremendous versatility ability to multiply rapidly, and, for some microbes, an incredible potential to mutate.

A Wealth of Knowledge

Prior to 1675, nothing was known of microorganisms. It was the invention of the microscope by the Dutchman Antonie van Leeuwenhoek that allowed us to see these previously invisible "animalcules" as he called them. With the light microscope he was able to see bacteria and larger microorganisms. Although there was evidence that smaller microorganisms existed and caused disease in humans and other animals, it was not until the development of the electron microscope in 1939 that viruses could be seen. Since then, our understanding of the classification diversity and pathogenic potential of the viruses, bacteria, fungi, and protozoa that infect and coexist with humans has increased exponentially Alongside this knowledge, it is important to stress that new pathogens continue to emerge and old pathogens reemerge or evolve to become even more pathogenic.


VIRUSES


Viruses are the most primitive of the microbes. They can have no existence independent of their host. To reproduce themselves,they must first attach to the host cell. This they do by recognizing the particular shape of a structure on the cell, which acts as a receptor. If this receptor is not present, the cell cannot be infected.

Once the virus has attached to the host cell it enters it and "uncoats." By various mechanisms, it then shuts off the host cell's normal biosynthetic pathways and subverts them into making new copies of the virus. These assemble either as large clusters inside the cell or at the cell membrane and are released by the cell bursting open, or by the virus "budding" from the cell surface. Each infected cell can release thousands of new virus particles, which then go on to infect other cells. This makes viruses "obligate intracellular parasites."

Finally, the host cell dies. If this process affects lots of cells, tissue damage which can be permanent or fatal.

However, the infected cells are not passive, but cry for help by releasing small molecules called cytokines and chemokines to recruit our immune system to the site of the infection. This contributes to the symptoms of infection, such as fever, and, in the respiratory tract for example, partial blockage of the airways and the production of sputum.

There are several consequences resulting from the virus only being able to reproduce itself inside a host cell. Firstly once it has been excreted from the host, virus numbers fall (this is not the case for most bacteria which are free-living). Secondly because the virus uses our synthetic machinery to reproduce itself, designing effective drugs to inhibit viruses is very difficult and this is why we have so few antiviral drugs.

Virus Characteristics

Viruses are very small, ranging in size from 25 to 350 nanometers (nm) in diameter. Some just have a protein coat, a "capsid," but others have an envelope of "lipid" surrounding the capsid. This envelope is usually acquired as the virus buds from the cell it has infected and is thus derived from the host cell membrane. In general, the viruses that have envelopes survive less well in the outside world (the environment) and are easily destroyed.

Viruses are unusual in that for some, the genetic code is of ribonucleic acid (RNA) and for others it is deoxyribonucleic acid (DNA), but never both, unlike humans. The shape of the capsid for some viruses is helical (spiral) and for others, it is a structure with 20 "facets." These characteristics make it possible to classify viruses into different families, genera, and species to provide a universally accepted scheme for naming and describing new viruses.


BACTERIA


Although some bacteria are obligate intracellular parasites like viruses, most are free-living and, providing they can scavenge enough food, will keep on reproducing. They reproduce by binary fission, each bacterium dividing to produce two identical offspring (progeny). Under optimal conditions, they can divide every 20 minutes so that after 18 -- 24 hours, one bacterial cell can divide to produce up to 10 million descendants. To put this in context, the world's human population is around 5,000 million.

Bacteria Characteristic

Bacteria range in size from 0.5 to 1 micron (µ) in diameter to 3 -- 15 µ long. Their classification is based upon their shape, staining characteristics, growth characteristics, biochemical properties and, increasingly on their genomic make up. Bacteria can be rod shaped (bacilli), spherical (cocci), or spiral (spirochaetes). The bacilli can be straight or bent. Cocci can be arranged in pairs (diplococci), in clusters (staphylococci) or in long chains (streptococci). Some bacteria, such as Bacillus anthracis (which causes anthrax), produce survival packages called spores. These allow the bacteria to survive under harsh conditions such as drying, heating, and irradiation, where non-spore producing bacteria are killed. When conditions improve, the spores germinate and the bacterium reproduces itself.

In 1895, Dr. Christian Gram, a Danish physician, devised a staining technique that is still used today to classify bacteria. Gram-positive bacteria, which stain blue, have one cell membrane and a thick cell wall. Gram-negative bacteria, which stain pink or red, have two cell membranes and a thinner cell wall.

Some bacteria are killed in the presence of oxygen. These are called obligate anaerobes. Other bacteria (facultative anaerobes) can grow whether oxygen is present or not.


FUNGI


The fungi form a large kingdom, but only a small number are pathogenic for humans. Fungi differ from bacteria in several ways. They are "eukaryotes," meaning that they have chromosomes in a nucleus and have internal structures called organelles. They are usually bigger than bacteria, more likely to be branching, and reproduce both sexually and by binary fission. Some, such as Candida albicans, remain as single cells, whereas others form large branching networks called mycelia.

Fungi can be subdivided into those that cause superficial infections (such as dermatophytes which cause an infection on the skin), those that cause infections in the tissues beneath the skin (subcutaneous), and those that disseminate around the body (systemic). They range from single-celled yeasts, such as Candida albicans (mentioned above) to large multicellular organisms with a sex life, such as those causing ringworm and athlete's foot. They have a cell wall and range in size from 5 µ to a few millimeters (mm) long.


PROTOZOA


The protozoa form a large kingdom of eukaryotes consisting of single-celled organisms. They have nuclei and intracellular organelles but no cell wall. These are single-celled organisms and begin to resemble human cells. Some, such as Entamoeba histolytica, reproduce by binary fission but others, such as Cryptosporidium parvum or Plasmodium falciparum, have a sexual phase of reproduction. They are quite fragile and those that are shed into the external environment usually produce hardy thick-walled cysts. Only a minority of the protozoa are pathogenic for humans. They range in size from 5 to 20 µ.


MULTICELLULAR PARASITES


Nematodes (round worms), trematodes (flukes), and cestodes (tapeworms) consist of large numbers of cells organized together. Some are pathogenic for humans.


WHAT IS NORMAL?


It is estimated that a human adult is less than 10 percent human. An adult comprises some 100 million million cells but fewer than 10 million million are human. The remaining 90 percent are the viruses, bacteria, fungi, protozoa, worms, and insects that make up our normal flora.

However, normal flora are not uniformly distributed throughout the body Microorganisms are found on exterior body surfaces, such as the skin and conjunctivae, and internal body surfaces that communicate with the exterior, such as the mouth, gastrointestinal tract, urethra, and vagina. The blood, organs such as the brain and liver, and respiratory tract below the vocal cords are normally sterile and finding bacteria in these areas usually indicates infection.

Bacteria, Viruses, Fungi, Worms

Bacteria make up the largest part of our normal flora. For example, half the wet weight of feces is made up of bacteria. It is estimated that there are approximately 1 million million bacteria per gram of feces. Some of the bacteria we carry can cause disease if they move from their normal colonization sites. For example, up to 50 percent of us carry Neisseria meningitidis in our throats. If it moves to the bloodstream or brain, it causes meningococcal septicemia or meningitis respectively.

Viruses can be found in the absence of infection; for example, once we have been infected by any one of the eight human herpes viruses, that virus will stay in our cells for life. Retroviruses make up three percent of our chromosomes and play an essential role in rearranging and activating our genes.

All of us carry fungi on our skins and yeasts such as Candida albicans can be found in the mouths and intestines of a large proportion of humans. We also carry protozoa such as Trichomonas oralis and Entamoeba coli in the mouth and colon respectively.

We can even be hosts for worms such as the threadworm (Enterobius vermicularis) and whipworm (Trichuris trichiura), without knowing it. We all also act as hosts for insects such as the follicle mite (Demodex follicularis), which lives in hair follicles, and the louse (Pediculus bumanus).


ROLE OF NORMAL FLORA


Our normal flora is for the most part for our benefit, but occasionally it can cause problems. For example, if a patient has an operation on the colon, the colonic bacteria, in particular anaerobic bacteria such as Bacteroides fragilis, spill into the peritoneum and cause peritonitis and abscesses. This risk can be minimized by giving antibiotics to cover the operating period.

The lack of normal flora or the wrong flora, however, poses greater problems. In experiments, if rodents are maintained without a normal flora, their immune system does not develop well and they are less able to mount a defense against invading pathogens. In addition, normal intestinal flora is important in the proper development of the gut. In germ-free rats, the intestine is greatly decreased in weight and surface area and the liver is decreased in size. Normal flora is also important in synthesizing some vitamins, for example vitamins K and the B complex.

Finally it is well recognized that perturbations of the intestinal flora, for example during treatment with certain antibiotics, will cause diarrhea. So overall, the normal flora is beneficial and its absence is detrimental.