Infection Control Today - 09/2003: Gram-negative and Gram-positive Bacteria

Gram-negative and Gram-positive Bacteria
Can’t Live With ‘Em, Can’t Live Without ‘Em

By Kathy Dix

Gram-negative and gram-positive bacteria both make up a significant part of the body’s defense system when they reside in the appropriate settings; taken out of their comfort zones, they can be fatal.

Bacterium, a one-celled organism without a true nucleus or cell organelles, belonging to the kingdom Procaryotae (Monera)¹

Bacteria can be classified as normal flora or as pathogens. As normal flora in their indigenous locales, they are benign, assisting in digestion and producing nutrients, but if the host is immunocompromised or the bacteria make their way to new territory, chaos can result.²

In the lab, it is possible to distinguish between gram-negative and gram-positive bacteria by use of gram staining. The Gram stain stems from the work of Danish physician Hans C.J. Gram. Gram was a pharmacologist and pathologist fascinated by botany, which led him to the microscope and the beginnings of pharmacology. In 1884, Gram published his findings that bacterial cells would “take up” and retain specific stains. Gram-positive bacteria retained the color of Gentian violet; gram-negative bacteria bleached. Another pathologist, Carl Weigert, later discovered that gram-negative bacteria retained stain from safranin.³

“The different way they stain ... has a lot to do with the composition of their cell wall on whether or not they stain positive or negative,” says Dennis Stevens, MD, professor of medicine and chief of infectious diseases at the Department of Veterans Affairs in Boise, Idaho.

Gram-negative organisms include salmonella, shigella, escherichia coli, and pseudomonas; gram-positive organisms include staphylococcus, streptococcus, clostridium and anthrax.

Stevens explains that the different types of bacteria have different types of cell walls. “They’re both pretty impermeable, but the gram negatives have as some of their major virulence factors a component of the cell wall called endotoxin or lipopolysaccaride,” Stevens says. Asked why bacteria would have evolved into two distinct types, he replies, “I imagine it’s just a matter of evolution and specialization of organisms. You could focus on the gram-positive versus the gram-negative, but you could ask the same question of why are some organisms round and others square and others long? It’s just a matter of that they evolved in particular niches.”

Gram-negative bacteria can be found most abundantly in the human body in the gastrointestinal tract, he says, which is where salmonella, shigella, e. coli and proteus organelli reside. Gram positives may also be found there, but also can reside on mucous membranes such as mouth, vagina or the skin.

Regardless of the type of cell wall, “most of them will do fine in a moist environment at body temperature, specifically those organisms that cause human disease,” Stevens continues. “There are other organisms that grow in hot geyser pools of 200 degrees at acid pH, and then there’s other organisms that live in the desert. But the ones that tend to cause human disease tend to like conditions in or on the human body, so that’s usually modest temperatures like 37 degrees and a reasonable amount of humidity.”

Some antibiotics are considered “broad-spectrum” because they can kill many microorganisms, often both gram-negative and gram-positive bacteria. More narrow-spectrum antibiotics can focus on just gram-negative or just gram-positive, while other antibiotics are specific to one type of bacteria.

“The microbial world is very diverse,” Stevens affirms. “There are some antibiotics that have good activity against both; an example of that would be miripinem or imipenem, and then there are other antibiotics that only have activity against gram-negative organisms, like aztrianam, and others that have activity only against gram-positive organisms, like vancomycin or lanazolid.” Vancomycin’s mechanism of action is to interfere with cell wall synthesis and it only affects the kind of cell wall gram-positive organisms have, Stevens says.

“Then there’s other antibiotics, like chloramphenicol, tetracycline, some of the fluoroquinolones like Levaquin, that have good activity against both gram positives and gram-negatives. There’s a difference in philosophy; I think it depends a little bit on the seriousness of the infection.

If it’s a mild or moderate infection, I think people will try to make a diagnosis, based on clinical knowledge and experience, and try to guess whether it’s going to be a gram-positive or gram-negative and use more specific treatment. The sicker the patient is, the more likely they’re going to get antibiotics that cover the whole spectrum,” he adds.

Broad-spectrum antibiotics, although impressive in their ability to kill many microorganisms, are still problematic in many ways. “They are more likely to induce resistance against a whole variety of microorganisms; they’re also more likely to cause diarrheal diseases and things like clostridium difficile, which is kind of an overgrowth of organism in the void created by broad-spectrum antibiotics,” says Stevens.

The choice between broad-spectrum and narrow-spectrum antibiotics depends on the severity of the illness. “For mild and moderate infections, I think usually clinicians try to make a specific diagnosis and use a narrowspectrum antibiotic,” Stevens affirms.

The sicker the patient, the more likely he will receive a broad-spectrum antibiotic. Physicians are loathe to wait on lab results when a patient is extremely ill; in those cases, it is better to risk antibiotic resistance than death. “In people who have shock or a fulminant kind of process, they’re either going to get an agent that has broad-spectrum (abilities) or they may get multiple antibiotics to cover the possibility of both gram-positive and gram-negative. You may end up using a combination because the patient is at death’s door; you don’t want to have a therapeutic misadventure,” he adds.

“For example, let’s say an otherwise healthy woman comes to the clinic and she has frequency and burning when she passes her urine. It’s likely this is going to be caused by the gram-negative organism e. coli. So you’re going to use an antibiotic that’s effective against e. coli. If somebody comes in with a big abscess in their skin, it’s more likely to be staph, so you’d prescribe an antibiotic against staph and you wouldn’t worry about the gram negatives. We’re talking about community-acquired sorts of things now.

“To shift gears to a more complicated situation, in the hospital setting, if a patient is compromised because he has leukemia or cancer and is on chemotherapy and he’s been in the hospital, then it’s more likely to be hospital-acquired organisms and they tend to be more resistant. If this patient is very ill, they’re very likely to get multiple antibiotics or an antibiotic with a very broad spectrum. It’s still important to culture the patient and make sure that you have the right diagnosis, and then once sensitivities of the organism are available, you can polish the broad spectrum down to a single agent,” continues Stevens.

If you’re dealing with a more sensitive bacteria that’s gram-negative, you’d use an antibiotic that is more specific for gram-negative and if you don’t isolate gram-positives, then you’d stop the coverage for the gram-positive,” he adds. “That tends to work out better in terms of the development of the whole burden of antimicrobial resistance in hospitals. I think we’re all much more cognizant of that now than people were five or six years ago, where there were so many good antibiotics, people probably used them with less discrimination than they have to today.”

Some bacteria are particularly onerous; clostridia was the reason autoclaves were invented, Stevens says. “The autoclave — which is just a big pressure cooker — generates essentially boiling temperatures at high pressures, and that’s necessary to kill clostridium tetani, clostridium botulinum, clostridium perfringens, because those organisms make spores that are very difficult to kill. Bacillus anthracis also makes spores; the spores themselves are what were used to mail all those letters with, and they’re very difficult to kill. You basically have to autoclave them for half an hour to an hour to kill the spores. [For] people who deal with sterilizers, the gold standard is to test the autoclave for its ability to kill bacillus spores. Now, we don’t use anthracis to test, but we do use spore strips of bacillus subtilis or bacillus sirius, which is a nonpathogenic bacillus that makes spores. Most infection control committees require that in-hospital sterilization techniques are at least sufficient to be able to kill that kind of organism.”

Bacteria are not just differentiated by gram-positive or gram-negative; they are also classified as aerobic — those that grow in the presence of oxygen — or anaerobic — those that grow in its absence. There are also organisms that can grow in both environments; these are known as facultative aerobes; these prefer an anaerobic environment but have adapted to living and growing in an oxygenated setting.

“It’s these anaerobic gram-positive spore-forming bacteria that are the big problem, and the reason that the autoclave is necessary,” Stevens clarifies. “Because back in the days before that, there used to be surgical wounds with tetanus from the spores from nonsterilized surgical equipment, or gas gangrene from lack of being able to kill the spores themselves.”

The anaerobic gram-positive spore-forming bacteria are so feared because they produce such potent toxins. “It doesn’t take many organisms to cause an entire human being to be totally paralyzed,” adds Stevens. “With tetanus, all the muscles contract. Clostridium botulinum produces a toxin that causes all the muscles to relax and they can’t contract so you’re paralyzed. Those (anaerobic gram-positive organisms) are the ones that cause gas gangrene, tetanus, botulism. The only clinically significant aerobic gram-positive spore-forming bacteria is anthrax — bacillus anthracis.”