Bacterial infections

What are bacterial infections?

For an infectious agent, infectivity refers to the proportion of exposed persons who become infected. Pathogenicity refers to the proportion of infected individuals who develop clinically apparent disease. Virulence refers to the proportion of clinically apparent cases that are severe or fatal. The disease process may never progress to clinically apparent illness, or result in illness that ranges from mild to severe or fatal. This range is called the spectrum of disease. Ultimately, the disease process ends either in recovery, disability or death (see Figure 2).

A bacterial infection is classified according to the site of infection, and sometimes also the source of infection (see Figure 1). Some examples include:

  • Skin infections
    • Acute bacterial skin and skin structure infection (ABSSSI)
  • Lung infections
    • Community-acquired oneumonia (CAP)
      • Probably Pneumococci, haemophilus influenzae, mycoplasma)
    • Hospital-acquired/ventilator-associated pneumonia (HAP/VAP)
      • Probably S.aureus, P.aeruginosa, Enterobacteria
      • Pseudomonas not in gut, it is found in shower heads or sinks -> can only infect after antibiotic treatment, when gut flora is compromised.
  • Intra-abdominal infection (IAI)
  • Blood-stream infection (BSI)
  • Urinary-tract infection (UTI)
Figure 1: Overview of bacterial infections. Typical pathogens are listed for each site of infection.

Sepsis

Sepsis is a special life-threatening state. Occurs rarely in UTI, mostly in lung infections.

The SOFA-score is often used to assess organ failure in septic patients.

Septic chock

Septic chock is a development of sepsis that is even more severe.

Symptoms

Periods of disease

The progression of an infectious disease can be divided into five periods, which are related to the number of pathogen particles and the severity of signs and symptoms (Figure 2). Infectious diseases can be contagious during all five of the periods of disease.

Periods of disease.
# Period/Phase/Stage Description
1 Incubation/latency Pathogen replicates. No signs or symptoms.
2 Prodromal (subclinical disease) Pathogeon replicates. General signs and symptoms. Immune system active.
3 Illness (clinical disease) Most severe and characteristic signs and symptoms. Usual time of diagnosis early in this period.
4 Decline Pathogen particle-count declines. Signs and symptoms improve. Immune system possibly weakened—suceptibility to secondary infection.
5 Convalescence Patient return to normal function. Signs and symptoms resolve. Some permament damage might have occured.

Stages might overlap. The onset of symptoms marks the transition from subclinical to clinical disease.

Figure 2: Periods of disease. This particular figure shows a complete recovery after illness. One can also imagine cases where the severity of symptoms never completely go back to baseline (a disability), or a case where the curves are truncated earlier in time (a death). Indeed, the time-scale is arbitrary here, as is the distance in time between the different periods. The periods of disease could even overlap in some diseases, adding to the complexity.

Diagnosis

Figure 3: Typical plasma concentration-time profiles of different biomarkers of bacterial infections. Adapted from Meisner M., (1999) [1]
  • Biomarkers not specific
    • 3 markers + WBC (but unspecific)
      • CRP (comes late)
      • PCT (quicker than CRP)
      • IL-6
  • Microbroth
    • Accurate + expensive
  • Vitek (turbidity), (will be replaced by MALDI-TOF?)
    • Not as accurate as microbroth

How can it be treated?

“Hit hard and hit early” -Paul Erlich

In almost all cases, an empirical treatment is used first. There no time to wait for culture results. The “right” treatment gets 90% success, “wrong” treatment gets 60% success (known as the “90/60 rule”).

Most infections can be treated in 5–7 days.

What we want to know before treating an infection

  1. Probable infection source
    • History
    • Examination (lab tests, X-ray, CT-scan)
  2. Probable bacteria & antibiotic susceptibility (see e.g. Figure 1)
    • Medical training
    • Local epidemiology
    • Resistance rates: How many percent are resistant?
  3. Individual resistance risk
    • Previous infection with resistant bacteria
    • Recent travel
    • Hospitalization or antibiotics
  4. Other considerations
    • Allergy
    • Other drugs (interaction risk)
    • Co-morbidities
    • PK
  • Clinical dogmas: Foreign body infection -> biofilm
  • Immunosuppressive

After initial treatment

  • Treatment re-evaluation after 3 days
    • 48 hours needed to evaluate antibiotic efficacy
    • The efficacy of antibiotic treatment is judged by signs and symptoms (Are you feeling better than yesterday?), and the course of inflammatory markers (WBC, CRP, PCT)
  • Empiric treatment should ideally be switched to targeted treatment once culture results are available (2–3 days)

References

[1]
Meisner M. Procalcitonin: Erfahrungen mit einer neuen Meßgröße für bakterielle Infektionen und systemische Inflammation. J Lab Med 1999;23:263–72. https://doi.org/10.1515/labm.1999.23.5.263.