25.2 Bacterial infections of the circulatory and lymphatic systems

Learning objectives

• Identify and compare bacteria that most commonly cause infections of the circulatory and lymphatic systems
• Compare the major characteristics of specific bacterial diseases affecting the circulatory and lymphatic systems

Bacteria can enter the circulatory and lymphatic systems through acute infections or breaches of the skin barrier or mucosa. Breaches may occur through fairly common occurrences, such as insect bites or small wounds. Even the act of tooth brushing, which can cause small ruptures in the gums, may introduce bacteria into the circulatory system. In most cases, the bacteremia that results from such common exposures is transient and remains below the threshold of detection. In severe cases, bacteremia can lead to septicemia with dangerous complications such as toxemia, sepsis, and septic shock. In these situations, it is often the immune response to the infection that results in the clinical signs and symptoms rather than the microbes themselves.

Bacterial sepsis, septic and toxic shock

At low concentrations, pro-inflammatory cytokines such as interleukin 1 ( IL-1 ) and tumor necrosis factor-α ( TNF-α ) play important roles in the host’s immune defenses. When they circulate systemically in larger amounts, however, the resulting immune response can be life threatening. IL-1 induces vasodilation (widening of blood vessels) and reduces the tight junctions between vascular endothelial cells, leading to widespread edema . As fluids move out of circulation into tissues, blood pressure begins to drop. If left unchecked, the blood pressure can fall below the level necessary to maintain proper kidney and respiratory functions, a condition known as septic shock . In addition, the excessive release of cytokines during the inflammatory response can lead to the formation of blood clot s. The loss of blood pressure and occurrence of blood clots can result in multiple organ failure and death.

Bacteria are the most common pathogens associated with the development of sepsis, and septic shock. S.P. LaRosa. “Sepsis.” 2010. http://www.clevelandclinicmeded.com/medicalpubs/diseasemanagement/infectious-disease/sepsis/. The most common infection associated with sepsis is bacterial pneumonia (see Bacterial Infections of the Respiratory Tract ), accounting for about half of all cases, followed by intra-abdominal infections ( Bacterial Infections of the Gastrointestinal Tract ) and urinary tract infections ( Bacterial Infections of the Urinary System ). D.C. Angus, T. Van der Poll. “Severe Sepsis and Septic Shock.” New England Journal of Medicine 369, no. 9 (2013):840–851. Infections associated with superficial wounds, animal bites, and indwelling catheters may also lead to sepsis and septic shock.

These initially minor, localized infections can be caused by a wide range of different bacteria, including Staphylococcus, Streptococcus , Pseudomonas , Pasteurella, Acinetobacter, and members of the Enterobacteriaceae. However, if left untreated, infections by these gram-positive and gram-negative pathogens can potentially progress to sepsis, shock, and death.