RT- Relevance and Incidence
The respiratory tract serves as the main pathway for pathogenic microorganisms to enter. Microorganisms replicate in the respiratory tract before spreading throughout the body. Acute lower respiratory tract infection (RTI) constitutes a pervasive and persistent health problem. RTI is common among children below the age 5 with pneumonia remaining the major disease. According to reports, childhood pneumonia is relatively higher (0.29 episodes per child-year) in developing countries than those in developed countries (0.05 episodes per child-year). Streptococcus pneumonia is the leading bacterial cause in the world today; identified in 40 to 50 percent of pneumonia cases.

Except for zinc supplementation and oxygen therapy for severe pneumonia, not much of clinical amelioration has been achieved in the last 2 decades. Researchers at the Board of Science and Technology for International development have said that more research on the modulating effects of immune deficiency and nutritional effects on inflammatory response could offer realistic opportunities on the disease outcome. Therefore, it is necessary and important to have potential preventive methods to improve the host defense system against pathogen challenges, especially in immune-compromised hosts.

Lactic acid bacteria (LAB) are long used in the production of probiotic foods and have the ability to stimulate the immune system. With the advances in molecular biology of LAB, the development of recombinant strains that express antigens from different pathogens have proved effective in inducing immunity. Thus, probiotics represent a key alternative reinforcing the defenses of the host against respiratory infections.

RT Infections in Immunocompetent Hosts
Most of the earlier research concerned with probiotic-mediated improvement of immune response focused primarily on gastrointestinal (GI) tract pathogens. Recent studies have begun emphasizing probiotics and its potential on stimulating the common mucosal immune system thereby offering protection to other mucosal sites. Researchers found that mice, which were virus-vaccinated, fed with bacterial strain YIT4064 (Bifidobacterium breve) showed marked increase in anti-influenza IgG antibodies (serum) and a higher survival-rate against the challenges of intranasal influenza when compared with virus-vaccinated/control-fed mice.

Few researchers also reported that when Lactobacillus casei strain Shirota was orally administered to aged mice, their cellular immune system got activated thereby reducing influenza virus titer in upper RT. Further, the researchers also reported that infant and neonatal mice when treated with L.casei strain showed improved pulmonary natural-killer (NK) cell activity and IL-12 production improving resistance against influenza infection in upper RT.

Lactobacillus casei CRL431, a powerful probiotic strain, and commercial yogurt (simulated) showed improved protection against the challenge (aerosolized) with Pseudomonas aeruginosa using the model with young mice. In treated yogurt-fed mice, there was a marked increase in the clearance rate of pathogen from the lungs. The phagocytic capabilities of alveolar macrophages showed up-regulation along with an increase in the bronchoalveolar fluid IgM and IgA levels the total serum level were seen. Also, in adult immunocompetent mice, enhanced immune response was seen when challenged with Streptococcus pneumonia showing immune-stimulating properties of LAB were dose- and strain-dependent.

When L.casei CRL431 strain, L.casei CRL1505, probiotic strain isolated from goat milk, and Lactococcus lactis NZ9000 strain (used for heterologous protein expression) were administered orally showed enhanced clearance rates of Streptococcus pneumonia in blood and lungs. Further, there was an improvement in the survival rate of infected mice by reducing lung injuries.

RT Infections in Immunocompromised Hosts
The effect of probiotic yogurt or L.casei CRL431 on the recovery of adaptive or inherent immune responses against Streptococcus pneumonia RTI in mice that were protein malnourished was determined. Yogurt or L.casei administration during repletion diet increased the normalization in the function and number of phagocytes. Further, the antibody response improved against pneumococcal infection mediated by various cytokines that were induced by L.casei was also detected. Early normalization of systemic humoral and local immune responses against pneumococcal infections in malnourished mice saturated with a balanced-conventional diet.

LAB and RT Pathogens as the Delivery System of Antigens
Respiratory tract infections cause high rates of mortality and morbidity. Probiotic applications in the respiratory tract help in the improvement of life and quality of infected populations. Beyond the advances in the production of live LAB vaccines that protect against respiratory pathogens, the complete potential of recombinant LAB still needs some good understanding. In addition, studies are being conducted on the mechanisms involved in mucosal protection in order to emphasize the difficulty and challenge involved in the task of combining adjuvants and vector vaccines.

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