4.2  Environmental monitoring and infection control applications
In addition to foodborne and waterborne pathogens, the detection of environmental pathogens is also an important aspect of healthcare. For example, diseases associated with environmental pathogens are one of the leading causes of death in low-income economies (WHO, 2018a). For example, malaria was reported to cause an estimated 435,000 deaths in 2017 (WHO, 2018b). Environmental pathogens are microorganisms that typically spend a substantial part of their lifecycle outside human hosts, but when introduced to humans through contact or inhalation cause disease with measurable frequency. Thus, environmental pathogens are often targets in medical diagnostics applications. However, here, we choose to distinguish environmental monitoring applications, which require pathogen detection in the environment (e.g., in air or on surfaces), from medical diagnostics applications, which require detection in body fluids. Thus, the distinction is based on the matrix in which the pathogen is present. Similar to food and water safety applications, which require biosensors capable of analyzing pathogen-containing complex matrices, such as a water or food matrix, environmental pathogens are present in multiple types of matrices. While environmental pathogens can enter the body through direct physical contact, they can also be transmitted through aerosols or interaction with organisms that serve as vectors for the infectious agent, such as mosquitos in the case of Plasmodium falciparum (the infectious agent associated with malaria). Thus, the detection of environmental pathogens often requires analysis of matrices, such as air, and objects, such as the surfaces of biomedical devices or objects within healthcare facilities, that are present in the human environment (Lai et al. 2009).
Several comprehensive reviews have been provided on the detection of environmental pathogens (Baeumner, 2003; Justino et al. 2017). Here, we describe the most common environmental pathogens found both in and outside of clinical settings. Common environmental pathogens in a non-clinical setting include Legionella spp., which cause Legionnellosis, Mycobacterium tuberculosis, which causes tuberculosis, and Naegleria fowleri, which causes amoebic meningitis. In addition to bacteria and protozoa, fungi, nematodes, and insects are also environmental pathogens. Common environmental pathogens in clinical settings associated with healthcare-acquired infections include drug-resistant and multi-drug resistant (MDR) pathogens, such as Clostridium difficile (CD) (Hookman and Barkin, 2009), which causes CD-associated diarrhea and antibiotic-induced colitis, and methicillin-resistant S. aureus (MRSA), which causes severe infections in various parts of the body, including the urinary tract (Gordon and Lowy, 2008).
The infectious dose of environmental pathogens also varies by orders of magnitude depending on the pathogen as well as age and health of the individual. For example, the infectious dose of CD is less than 10 spores, while that of MRSA is greater than 100,000 organisms (Schmid-Hempel and Frank, 2007). While it is possible to obtain antibodies for foodborne and waterborne pathogens, it can be challenging to obtain antibodies for various environmental pathogens, including protozoa and nematodes. Thus, traditional bioanalytical techniques, such as PCR, are often utilized for the detection of environmental pathogens.
Similar to food and water safety applications, biosensor-based assays for environmental pathogen detection applications also utilize measurement formats that facilitate the analysis of liquids. However, they also require measurement formats for the detection of aerosolized pathogens. In addition to airborne transmission, environmental pathogens are transmitted by direct surface contact (similar to many foodborne pathogens), which is a significant mode of transmission in healthcare settings (e.g., of healthcare-acquired infections). Standardized guidelines for disinfecting and sterilizing the surfaces of medical equipment, assistive technologies, counters, and doors, among other surfaces, have emerged as an important aspect of infection control in modern healthcare facilities (Fraise et al. 2008). Thus, the detection of pathogens on the surfaces of biomedical devices and objects present in healthcare facilities is an important research area (Kramer et al. 2006; Weber et al. 2010). For example, bacterial contamination of inanimate surfaces and equipment has been examined as a source of intensive care unit-acquired infections, a global healthcare challenge, especially when caused by MDR pathogens (Russotto et al. 2015). Hospital-acquired infections are prevalent causes of morbidity in patients (Orsi et al. 2002). This problem has only been exasperated by the rise of MDR CD, as well as drug-resistant strains of Campylobacter, Enterococcus, Salmonella, S. aureus, and S. dysenteriae (Ventola, 2015). In addition to clinical pathogens, it is also of interest to detect pathogens in non-clinical settings (Faucher and Charette, 2015). Toxin-producing algae, such as cyanobacteria and sulphate-reducing bacteria, are also important targets for electrochemical biosensors associated with the prevention of water-based diseases.