A Non-invasive and Technically Non-intensive Method for Induction and Phenotyping of Experimental Bacterial Pneumonia in Mice

The overall goal of this experimental protocol is to provide a non-invasive and technically non-intensive procedure for inducing bacterial pneumonia in mice and the subsequent quantification of the in vivo host defense response in the lungs. This method can help answer key questions in the fields of infectious disease and immunology, about the genes and molecular pathways that control the host response to infection. The main advantage of this lung infection method is its technical simplicity.

Which allows even laboratories with little expertise in pulmonary procedures to study the pulmonary innate immune response. In a biosafety level two facility, thaw a glycerol stock of K.pneumoniae and transfer one milliliter of the bacteria into a 500 milliliter culture flask containing 50 milliliters of Tryptic Soy Broth. Incubate the suspension culture overnight at 37 degrees Celsius and 200 to 225 RPM.

The next morning, dilute one to five milliliters of the bacterial culture in a new flask containing 50 milliliters of Tryptic Soy Broth, and incubate the culture, in the shaker, at 37 degrees Celsius for an additional 2.5 hours to achieve log phase. At the end of the second incubation, transfer one milliliter of bacteria from the second culture to a 1.5 milliliter tube for centrifugation. Remove the supernatant without disturbing the palette and gently resuspend the bacteria in one milliliter of sterile saline for three separate washes.

After the third wash, resuspend the bacteria in another milliliter of sterile saline, and set up log-wise serial dilutions of the inoculum. Measure one milliliter of the one to 10 and one to 100 dilutions in disposable cuvettes, in duplicate, to determine the OD600 of the washed K.pneumoniae. An OD600 of 1.0 is roughly equivalent to four to seven times 10 to the eighth CFUs per milliliter.

After calculating the concentration of the bacteria in each dilution, resuspend the desired inoculum CFU dose in 50 to 60 microliters of sterile saline per eight to 12 week old recipient animal. To deliver the bacteria to the animals, draw up the dosing inoculum into a P200 pipette tip, and place the anesthetized mouse in a semi-recumbent supine position, suspended by the maxillary incisors, from a rubber band stretched between the pegs of a slanted Plexiglas board. Using a pair of blunt, non-ridged forceps, gently pull the tongue to the side and deposit the dose into the animal’s oral cavity taking care to avoid inducing trauma to the tongue or the oral pharynx.

Keeping the tongue retracted, gently occlude the nose with a gloved finger, until the house inhales two to three times. When no liquid is visible in the oral cavity, transfer the mouse from the inoculation board to its cage, in the supine position, to prevent blockage of the nares while the mouse is recovering. Make a longitudinal cut just below the sternum.

Then, lifting the sternum with forceps, nick the diaphragm, allowing the lung to fall back into the chest cavity. Continue the incision up through the chest cavity on either side of the vasculature and up through the neck. When the trachea is visible, carefully cut through the middle of the neck and push the tissue to the sides to avoid damaging the surrounding vasculiture.

Now nick the trachea about one quarter of the way down from the head, and caudally insert a cannula attached to a one milliliter syringe pre-loaded with one milliliter of PBS into the trachea. Push the volume into the lungs, slowly, allowing all of the lobes to inflate. Then pull the volume back out with the syringe and pool the collected washes in a 15 milliliter tube, on ice.

At the appropriate experimental end point, collect the blood from the left ventricle and transfer it into a heparinized tube to avoid clotting. Then harvest all of the lung lobes in a 15 milliliter tube containing five milliliters of PBS, and the spleen in a 15 milliliter tube containing two milliliters of PBS. Next, use individual, disposable homogenizers per sample, to macerate the tissues on ice, followed by serial dilution of the tissue slurries.

Plate the dilutions, in duplicate, on separate sides of a single TSA plate, and allow the samples to, briefly, dry. Then invert the plates and culture the samples in a static 37 degrees Celsius incubator overnight, averaging the bacterial colonies from both sides of the plate and from each dilution the next morning. At 2000 CFU of K.pneumoniae, mice, typically, begin to demonstrate clinical symptoms 12 to 24 hours post infection.

Within 48 to 72 hours, many of the animals exhibit symptoms of illness and morbidity that are typically preceded by an average of 20 percent weight loss. An LD50 dose, however, allows the detection of both the relatively increased and decreased survival within the experimental group, and may be preferred for longer term studies. Lower respiratory tract infection with K.pneumoniae is characterized by a robust influx of leukocytes into the airway within six hours of infection, that peaks by 24 hours and is mostly represented by neutrophils.

Cytokines are detectable in the bronchoalveolar lavage fluid at both 24 and 48 hours post infection with K.pneumoniae, and provide insight into the lung microenvironment and its immune self-recruitment in response to bacterial invasion. The bacterial load in the lung, blood, and spleen, also dramatically increases between 24 and 48 hours of infection. Once mastered, the oropharyngeal aspiration method of lung infection can be done in just one to two minutes per mouse if it is performed properly.

While attempting this procedure, it is important to remember not to use an excessive aspiration volume. A 50 to 60 microliter volume typically works well for an eight to 12 week old mouse. Following this procedure, a variety of methods can be performed on the lung and peripheral tissues to allow the quantification of the immune response in pathogen clearance.

After watching this video you should have a good understanding of how to induce experimental bacterial pneumonia via oropharyngeal pathogen delivery and to determine the pathogen burden in mice. Don’t forget that working with microbes can be hazardous and that basic BSL-2 precautions and practices should always be taken while preforming this procedure.