On sepsis

Infections are infamous for taking a staggering amount of human lives over the centuries even in hospitals, particularly before germ theory of disease took over the miasma theory. But there was one man who understood infection far more than his contemporaries. He who introduced the practice of doctors washing their hands in between seeing patients was mocked, berated and driven to madness. His life ended after being lured into an insane asylum, beaten by guards as he tried to escape.

Ignaz Semmelweis presented empirical evidence showing that cleanliness reduced post-delivery mortality. Medical community of the time would hear none of it: if he couldn't explain himself in theoretical terms his points were invalid, they thought. Furthermore, implying doctors' own fault in introducing disease by being dirty was certainly perceived as an insult. Only several decades later did his views become understood and rooted in a new understanding of disease which shifted the blame from 'polluted air' or 'miasma' to germs.

When Louis Pasteur suggested that microbes responsible for gangrene might be eliminated by using certain chemical solutions, Joseph Lister began experimenting. His work resulted in a new protocol for working in the operating room which included clean clothes, gloves, and medical equipment sterilized by a 5% carbolic acid solution. As the germ theory of disease gained credibility and doctors realized that by not allowing free passage to bacteria one could markedly reduce infection rates, Lister's idea of sterile surgery became the norm.

One would think that 152 years after Semmelweis published his nominal ‘Die Aetiologie, der Begrif und die Prophylaxis des Kindbettfiebers’ (Etiology, Concept and Prophylaxis of Childbed Fever) and 146 years after Lister's Antiseptic Principle of the Practice of Surgery, death from infections in hospitals would be completely eliminated. Nothing is ever so simple, however. Enter sepsis.

Severe inflammatory response syndrome (SIRS) with a suspected or confirmed microbial etiology, sepsis, in its severe form contributes to over 250,000 annual deaths in North America and even more in Europe. A review by Greg S. Martin reveals that

"applying the consensus conference definition, rough estimates of fatality rates (the percentage of patients who die) are as follows:

Sepsis: 10–20%

Severe sepsis: 20–50%

Septic shock: 40–80%"

Despite the risk of death due to sepsis today being lower than before, higher incidence rates result in more people than ever dying from sepsis. It seems counter-intuitive in the age when all but a few infectious diseases remain as dangerous as they have been, to see more people dying from an infectious cause than before. Why is it this way? Understanding and increasing awareness of sepsis is crucial in this day and age for health-care professionals and general public alike.

Pathophysiology of sepsis is based on malfunctioning of the immune response and is therefore seen most often in immunocompromised patients, such as those suffering from AIDS or elderly people with underlying chronic morbidities. To put it simply, components of the immune system react vigorously by releasing pro-inflammatory molecules (cytokines, chemokines) in hope of containing an infection but end up harming the host rather than protecting it. It's not the whole story though; untimely death of leukocytes by apoptosis and/or necrosis and problems with coagulation have been noted to greatly contribute to sepsis. 

When bacteria (or other pathogens) are first noted inside our body, parts of them called Pathogen-Associated Molecular Patterns (PAMPs) bind to Toll-like receptors on leukocytes. Macrophages polarize to an M1 phenotype (pro-inflammatory) and release pro-inflammatory cytokines such as interleukin-1(beta), tumor necrosis factor (TNF), interleukin-6; mast cells contribute histamine.

Cytokines stimulate endothelial cells to produce adhesion molecules. White blood cells latch onto these molecules and pass through from the blood into infected tissue. Phagocytes (resident macrophages and travelling neutrophils) engulf pathogens and degrade them inside. If pathogens are numerous enough though they can escape phagocytosis. When some pathogens remain lying around cytokines will continue to be released and an inflammatory response will continue to escalate. 

With a serious enough infection it's possible that a response against pathogens will be so strong so as to become more deleterious to the host rather than the invader. For example, histamine is responsible for making blood vessels more permeable and therefore useful in getting white blood cells to infected tissues. However, during sepsis it is released systemically and causes the majority of blood vessels to become leaky, which results in severe hypotension. Therefore even if inflammation is detrimental it could be unwise to prescribe anti-inflammatory medication because it's necessary to battle infection. It can only be said that inflammation is causing damage and not whether this inflammation is called or uncalled for.

Adaptive immunity also plays a role. Antigens are presented to T cells by antigen presenting cells (APCs), among other things, which in turn stimulate CD4+ T cells to release cytokines and further develop an orderly immune response. There's often a decline in T cell number in septic patients, which is not unexpected especially in immunocompromised individuals. 

Increase in the rate of apoptosis could be responsible for this. More apoptosis was found in the spleen and thymus of septic patients. It was later shown that particularly CD4+ T cells and dendritic cells suffered most apoptosis. The degree of apoptosis was positively correlated with severity of sepsis and its outcome.
On another side of the spectrum, neutrophils show decreased rate of apoptotic death. Their infrequent apoptosis could contribute to organ damage by continuous release of toxic materials.

The last part of pathophysiology I'd like to discuss is coagulation. It was noted in the review that while dysfunctions of coagulation are present in sepsis, nothing beyond this can be said with certainty. What is accepted is that inappropriate intravascular fibrin deposition occurs. I'd like to briefly present what this means. 

Fibrin is the end product of coagulation which traps blood cells and constitutes a clot. In sepsis however, fibrin is produced quickly and markedly in small blood vessels. During this process small blood clots form. This causes a condition known as disseminated intravascular coagulation, DIC. These blood clots result in hypoperfusion to organs around which they form and often result in organ dysfunction since organs don't have the opportunity to remove waste and receive nutrients and oxygen. Furthermore, by using up all the clotting factors it leaves other places vulnerable to uncontrollable bleeding, mainly in the GI tract, respiratory system.

So what symptoms can such pathophysiology precipitate?

Symptoms of SIRS include

deviations from normal temperature (either hypothermia, <36C or hyperthermia, >38C) 

Heart rate >90 BPM 

Respiratory rate >20 per minute 

WBC count >12 K or <4 K per cubic millimeter 

Stepping up to a more serious stage, sepsis with one of more dysfunctioning organ system is called severe sepsis. For example, urine output decreases since filtration cannot occur properly in the kidney with hypotension due to excess fluid leakage from overly permeable blood vessels all over the body.
Liver dysfunction and altered mental status are also possible. 

Septic shock occurs when sepsis presents with hypotension unresponsive to given fluids. It is a life-threatening condition and the most serious complication of sepsis. The heart cannot keep working so cardiac output plummets, organs can't maintain homeostasis and myocardial infarction occurs, among other things, resulting in death.

Increased incidence of sepsis can be attributed to an aging population (particularly in the U.S. and Europe) with chronic underlying illnesses, ever increasing number of medical procedures performed and more time spent with catheters, escalating drug-resistance among infectious agents.

Treatment for sepsis is crucial and must be administered as quickly as possible. Antimicrobial agents, removal of the source of infection (i.e. medical equipment), also hemodynamic, respiratory and metabolic support must be applied depending on severity and symptoms. 

Once developed it is extremely difficult to combat, therefore prevention offers the best opportunity to reduce morbidity and mortality from severe sepsis and septic shock. Reduction of unnecessary medical procedures, using antiseptics and avoiding nosocomial infections is of utmost importance.

So perhaps it's not so strange that we face grave danger from sepsis. While even today doctors do not wash their hands as often as is required and nosocomial infections still arise in hospitals, sepsis is a killer unlike other infectious diseases. When we talk about an increasingly old age and number of medical procedures as causes of a disease, we acknowledge our achievements in other aspects of medicine. Just as many people didn't get cancer when average life expectancy of a person was around 40, perhaps increasing incidence of sepsis also shows us that we have reached a point in medical development when the greatest maladies are starting to be forgotten and we are being forced to focus our attention on conditions which were previously overshadowed by even more vicious killers.

P.S. Sepsis is a tough nut to crack. Recently it was shown that mouse models of sepsis are unhelpful when modeling sepsis in humans. Who knows what alterations to our currently accepted mechanism might be for us to discover in the future.

References: 

1. Stearns-Kurosawa DJ, Osuchowski MF, Valentine C, Kurosawa S, Remick DG. The pathogenesis of sepsis. Annu Rev Pathol. 2011;6:19-48. doi: 10.1146/annurev-pathol-011110-130327.
2. Martin GS. Sepsis, severe sepsis and septic shock: changes in incidence, pathogens and outcomes. Expert Rev Anti Infect Ther. 2012 Jun;10(6):701-6. doi: 10.1586/eri.12.50.
3. Longo et al. Harrison's Principles of Internal Medicine 18th ed.

Antibiotics remain elusive

Science moves fast. Nowadays we don't have to wait hundreds of years to correct erroneous understanding as was the case with the gospel of ancient masterminds like Claudius Galen. Currently scientists can report something novel and discordant when compared with prevailing views and within less than ten years themselves become a target for scrutiny from their keen, attentive colleagues.

Recently it was reported that previous findings which indicate an existence of a general mechanism by which antibiotics kill bacteria could be wrong.

In 2007 researchers discovered that there might be "A Common Mechanism of Cellular Death Induced by Bactericidal Antibiotics". Scientists thought they found a discrepancy in classifying antibiotics into categories by their mechanism of action and that rather they shared one common way of killing bacteria.

All antibiotics elicited an increase in "production of highly deleterious hydroxyl radicals in Gram-negative and Gram-positive bacteria which ultimately contribute to cell death" and that "bacteriostatic drugs do not produce hydroxyl radicals". Thus, researchers concluded, "all three major classes of bactericidal drugs can be potentiated by targeting bacterial systems that remediate hydroxyl radical damage, including proteins involved in triggering the DNA damage response".

Now, of course we know that reactive oxygen species (ROS) are nothing to be scoffed at. Any chemist will tell you that having a molecule floating around with an unpaired electron in its outer shell is one of the most dangerous situations a stable local environment can find oneself in. First contact will be the last contact: ROS will bind to its first victim molecule and disrupt the functionality of said molecule. Add several of those angry scavengers and you are certainly in trouble.

Of course, not all is black and white. Some believe that free radicals and ROS under certain conditions can be beneficial to the organism and its longevity. Nothing is yet certain, but the field is certainly not stuck in a blissful state of complete understanding.

Returning to the issue at hand, all would seem at least somewhat logical that by inducing hydroxyl radical production antibiotics clearly contribute to them destroying bacteria. However an aforementioned study in Science refutes this notion.

Researchers claim that they

"found no correlation between an individual cell's probability of survival in the presence of antibiotic and its level of ROS".

 By using a compound, thiourea, an "ROS quencher" they

"protected cells from antibiotics present at low concentrations, but the effect was observed under anaerobic conditions as well".

Since ROS are produced by aerobic respiration, the ability of an ROS quencher to protect bacteria from antibiotics in anaerobic conditions in which ROS cannot be produced indicates that ROS cannot be the deciding factor of bactericidal action.

To be even more sure, they marked cells with a fluorescent tag which showed ROS, separated those cells with highest amount of fluorescence (therefore, highest amount of ROS) from the lowest ones and both populations suffered equivalent cell death upon being bathed in antibiotics.

So the researchers concluded that

"There was essentially no difference in survival of bacteria treated with various antibiotics under aerobic or anaerobic conditions" and thus "this suggests that ROS do not play a role in killing of bacterial pathogens by antibiotics".

With this notion of general mechanism by which antibacterials operate severely damaged, we return to viewing each class as different. Some inhibit the synthesis of cell walls (beta-lactams like penicillin), others dampen the synthesis of nucleic acids (fluoroquinolones), proteins (tetracyclines), act as anti-metabolites (folate pathway inhibitors: sulfonamides) or disrupt membrane function (cyclic lipopeptides: daptomycin).

There is yet much to be said about antibiotics, most pressingly their overuse. Ever increasing bacterial resistance to antibiotics is one of the most pressing matters to be solved. This is exemplified by the recent "nightmare bacteria" which is spreading across US hospitals and is resistant to most powerful antibiotics. More importantly, it kills half of those it infects. Dr. Marc Siegel vocalizes the seriousness of this situation: ""To see bacteria that are resistant is worrisome, because this group of bacteria are very common. The more you use an antibiotic, the more resistance is going to emerge. This is an indictment of the overuse of this class of antibiotic [carbapenems]."

There is a growing awareness and desire to cut back on needless antibiotic usage among an increasing number of people. A piece in The Atlantic has one such anti-antibiotic enthusiast who may just be going a bit too far ("our 14-year-old son had never taken antibiotics"), but at least it shows an increase in understanding that an overly runny nose or a mild fever does not require an onslaught of broad-spectrum antibiotics.

One especially crucial point was made:

"Antibiotics are unnecessary for colds or bronchitis, even when they last longer than two weeks.  Colds and bronchitis often take more than two weeks to resolve, so if there are no signs of pneumonia, then antibiotics can be withheld safely."

Antibiotics are used each and every day and everyone has a general understanding of what they do. In spite of this they remain elusive and still baffle some researchers. They are useful and crucial for keeping the society healthy, however our need to shorten the duration of even the mildest of discomforts can lead to our inability to protect ourselves when the real enemy emerges.


References:

1. I. Keren, Y. Wu, J. Inocencio, L. R. Mulcahy, K. Lewis. Killing by Bactericidal Antibiotics Does Not Depend on Reactive Oxygen Species. Science, 2013; 339 (6124): 1213 DOI: 10.1126/science.1232688

2. Michael A. Kohanski, Daniel J. Dwyer, Boris Hayete, Carolyn A. Lawrence, James J. Collins. A Common Mechanism of Cellular Death Induced by Bactericidal Antibiotics. Cell - 7 September 2007 (Vol. 130, Issue 5, pp. 797-810). doi:10.1016/j.cell.2007.06.049

Homeostasis and immunity

For a first blog post I decided to talk about homeostasis, which is a dynamic regulation of a system's internal environment, and the immune system. The usual choices for this topic when talking about humans would be the nervous and endocrine systems, but I wanted to explore what the immune system has to do with maintaining homeostasis. Later posts will examine this relationship more closely, so as to present the topic with an incrementally increasing level of detail.

To begin with, briefly on homeostasis. The term encompasses a huge portion of nature and all of life. It can be thought of as what gives a system the ability to keep itself comfortable despite pressure from the environment.
Wherever we turn our attention, some sense of homeostasis always pops up, especially if you are trying to find it. It is present in cells as they use pumps to maintain a certain ion concentration, it is evident in an organism which protects itself from overheating by evaporating water from the skin, it is noticeable in society as countries try to maintain their independence and survival by diplomatically regulating both domestic and foreign affairs.

No matter where one looks, homeostasis can always be identified by locating the three main components: sensor, integrator and effector. 

First of all, there must always be something that disturbs the system, like a virus which wreaks havoc or has the potential to do so, or an elevation/fall of some vital parameter, like temperature.

The sensor notices that something is wrong and begins the homeostatic process by informing the integrator about the impending doom to the system's stability and ability to function. The integrator decides what is to be done and transmits commands to the effector which reacts accordingly to the stimulus and if everything goes right restores the normal status of a system.

Of course, even though this description implies that homeostatic mechanisms come into action only when a disturbance occurs, actually due to there being a whole lot of different parameters to be maintained, some mechanisms are always in action. For example the mentioned pumps work constantly by making sure that the intracellular fluid contains more potassium and is more negative than the extracelullar fluid which should be more positive and rich in sodium.

Now, the fine line between a sensor, integrator and effector is pretty clear when talking about the nervous and endocrine systems. Dominika Dabrowski has written a nice overview of thermoregulation: when the temperature drops "this stimulates skin cold receptors (increase in their activity) and cools the blood flowing into the skin. These signals are received by both the hypothalamic thermostat and higher cortical centers. The thermostat is also activated by the change in blood temperature. It initiates responses that promote heat gain and inhibits centers that promote heat loss".

We can immediately identify that the skin receptors are the sensors, the hypothalamus and cortical centers are the integrators. What's left are the effectors which respond to the hypothalamus by inducing involuntary muscle contractions (shivering) to produce heat, vasoconstriction to reduce heat loss, etc.

However, in the immune system, things get quite different and actually the mechanism is not so straightforward as mentioned in the general example. Pathogens are not like temperature. Temperature is just a certain value of a property, while various germs are unwelcome guests and their existence is an absolute value: you can either be free of germs, or infected with germs. There is no middle ground.

Therefore, it is not so unexpected to see some variability in the mechanism of homeostasis, even though the goal remains internal stability. There may be some generalized effectors which remove detrimental stimuli without them being sensed previously, and only after this general effector is breached, the sensor picks up the signal and starts a real, initiated response. The sensors could also be the effectors at the same time. Dendritic cells, for example, swallow the pathogens (effector action) and then present the pathogen on their surfaces so that it can be recognized (sensor action).


Talking about generalized effectors, it is obviously beneficial to be able to ward off unwanted micro-guests without expending any energy, or as little as possible. And since the general consensus the body makes is that "either you're me or you're against me" (the failure of following this consensus will be the topic of a later post), the body has barriers - generalized effectors - which block easy entrance of pathogens into the body. The skin envelops the body and wards of weaponless microorganisms, but they can still get inside the body via openings, such as mouth, nose, ears and gain access to sensitive internal environment of the respiratory or gastrointestinal system. However, the body won't be fooled so easily. The mucous membranes which are present in these areas produce mucous secretions, for example nasal mucus in the nasal passages. Mucus traps small particles such as dust, pollen, also bacteria and viruses which in the absence of mucus would be able to easily penetrate into the lower respiratory tract and other sensitive areas. So when you unpleasantly notice that your mucus is greenish or yellowish, realize that this is the first line of defense doing its job and trapping the bacteria or viruses within itself.

Unfortunately, this superficial defense is very limited and not so effective in maintaining homeostasis against any serious foe. That is obvious, since each year millions of people get colds and other sorts of viral, bacterial and fungal infections. Germs, like Rhinovirus, which remain on the periphery of the respiratory and gastrointestinal tracts cause mild infections like the common cold. Others, mainly bacteria, can induce moderate ones: pharyngitis, laryngitis, etc.
Pathogens can also penetrate the skin, for example, with the help of our misfortune of getting a splinter or a bruise. They can travel in company with ectoparasites, like Plasmodium falciparum does with Anopheles mosquitos and when the mosquito tries to get blood it inadvertently allows Plasmodium to enter the bloodstream and causes one of the most detrimental infectious diseases - malaria.

Fortunately, the immune system is not composed solely of the skin and some slimy mucus. Such external barriers represent only one part of the innate immunity, which together with the acquired immunity represent the way that our organism responds to pathogenic stimuli which threaten its homeostatic stability and sometimes its very existence.

So the immune system is directly linked with maintaining homeostasis both constantly, i.e. with a constant presence of barriers to pathogens, and also upon being induced, when those pathogens breach the general defenses and induce sensors to call for reinforcements.