Here, we describe ex vivo and in vivo methods for assessing bacterial dispersal from a wound infection in mice. This protocol can be utilized to test the efficacy of topical antimicrobial and anti-biofilm therapies, or to assess the dispersal capacity of different bacterial strains or species.
Biofilm-related infections are implicated in a wide array of chronic conditions such as non-healing diabetic foot ulcers, chronic sinusitis, reoccurring otitis media, and many more. Microbial cells within these infections are protected by an extracellular polymeric substance (EPS), which can prevent antibiotics and host immune cells from clearing the infection. To overcome this obstacle, investigators have begun developing dispersal agents as potential therapeutics. These agents target various components within the biofilm EPS, weakening the structure, and initiating dispersal of the bacteria, which can theoretically improve antibiotic potency and immune clearance. To determine the efficacy of dispersal agents for wound infections, we have developed protocols that measure biofilm dispersal both ex vivo and in vivo. We use a mouse surgical excision model that has been well-described to create biofilm-associated chronic wound infections. To monitor dispersal in vivo, we infect the wounds with bacterial strains that express luciferase. Once mature infections have established, we irrigate the wounds with a solution containing enzymes that degrade components of the biofilm EPS. We then monitor the location and intensity of the luminescent signal in the wound and filtering organs to provide information about the level of dispersal achieved. For ex vivo analysis of biofilm dispersal, infected wound tissue is submerged in biofilm-degrading enzyme solution, after which the bacterial load remaining in the tissue, versus the bacterial load in solution, is assessed. Both protocols have strengths and weaknesses and can be optimized to help accurately determine the efficacy of dispersal treatments.
The rise of antibiotic resistance worldwide is leading to a lack of antibiotic options to treat a variety of bacterial infections1. In addition to antibiotic resistance, bacteria can gain antibiotic tolerance by adopting a biofilm-associated lifestyle2. A biofilm is a community of microorganisms that are protected by a matrix of polysaccharides, extracellular DNA, lipids, and proteins3, collectively called the extracellular polymeric substance (EPS). As the antibiotic resistance crisis continues, new strategies that prolong the use of, or potentiate the efficacy of, antibiotics are sorely needed. Anti-biofilm agents are one promising solution4.
Amongst the different anti-biofilm strategies that have been proposed, the utilization of dispersal agents, which target different components of the biofilm EPS, are at the forefront of therapeutic development5. Glycoside hydrolases (GH) are one such class of dispersal agent. GH are a large class of enzymes that catalyze the cleavage of different bonds within the polysaccharides that provide structural integrity to the EPS. Our group, as well as others, have shown that GH can effectively degrade biofilms, induce dispersal and improve antibiotic efficacy for a number of different bacterial species, both in vitro and in vivo6,7,8,9,10,11.
With a growing interest in biofilm dispersal,it is important to develop effective methods that assess dispersal efficacy. Here, we present a detailed protocol for the treatment of biofilm-associated wound infections with a dispersal agent in mice, and the assessment of dispersal efficacy, in vivo and ex vivo. The overall goal is to provide effective methods that can be used with preclinical models to measure biofilm dispersal effectively and efficiently.
A murine surgical excision infection model was used in these studies to establish a biofilm-associated infection. We have used this model for over 15 years and published our observations extensively7,9,12,13,14,15,16,17,18,19,20,21. In general, this is a non-lethal infection model where bacteria remain localized to the wound bed and are biofilm-associated (bacteria seen in aggregates surrounded by EPS), setting up a chronic infection that lasts up to 3 weeks. However, if mice are immunocompromised (with Type 1 diabetes for example), they can become more susceptible to developing a fatal systemic infection in this model.
In this report, we provide protocols for assessing the dispersal of bacteria from a wound, both in vivo and ex vivo. Both protocols can be used to examine the efficacy of a dispersal agent and have their own strengths and weaknesses. For example, assessing dispersal in vivo can provide important, real-time information about the spread of bacteria to other parts of the body after dispersal, and how the host responds. On the other hand, assessing dispersal ex vivo may be more desirable for screening multiple agents, doses, or formulations, as the tissue can be divided into multiple sections that can be tested separately, thus reducing the number of mice required. When assessing multiple agents, we typically measure dispersal first in vitro as previously described 6,9,22. We then test the most effective ex vivo and reserve in vivo testing for a limited number of very promising agents.
This animal protocol was reviewed and approved by the Institutional Animal Care and Use Committee of Texas Tech University Health Sciences Center (protocol number 07044). This study was carried out in strict accordance with the recommendations in the Guide for the Care and Use of Laboratory Animals of the National Institutes of Health.
1. Preparing bacteria for mouse infections
NOTE: Here we describe infecting mice only with Pseudomonas aeruginosa. However, other bacterial species may be used to cause infection. Bacterial strains and materials are detailed in the Table of Materials.
2. Preparation of biofilm dispersal enzyme
NOTE: For this study we use a 10% solution of equal parts amylase and cellulase (5% of each), which will be referred to as "GH", for the dispersal treatment.
3. Experimental animals and preoperative setup
4. Dorsal full thickness excision surgery
5. In vivo dispersal treatment
NOTE: Here we describe administration of the dispersal agents by applying a series of 3 topical wound irrigation solutions (Figure 1). However, the protocol can be adapted for other types of delivery such as the application of gels, creams or dressings.
6. In vivo dispersal imaging and analysis
NOTE: If a luminescent strain of bacteria is utilized to initiate infection, an In Vivo Imaging System (IVIS) can be used to visualize dispersal from the wound bed.
7. Assessing dispersal by determining CFU
8. Ex vivo assessment of dispersal (Figure 4)
In this experiment, 8-10 week old female Swiss Webster mice were infected with 104 CFU of PAO1 carrying the luminescence plasmid pQF50-lux. As described above, an infection was allowed to establish for 48 h prior to administering 3 x 30 min treatments of either PBS (vehicle control) or 10% GH (treatment) to digest the biofilm EPS. Mice were imaged pre-treatment, directly after treatment (0 h) and at 10 h and 20 h post-treatment. Figure 2A and Supplemental Figure 1 show an established infection within the wound bed generating a bright bioluminescent signal. The dispersal of bacteria out of the wound bed can be visualized immediately after the GH treatment (0 h), but not after PBS treatment. It should be noted that in previous studies, we detected bacteria in the blood and spleen as early as 5 h post GH treatment7. Bacterial dissemination into the organs can also be seen by placing the mouse on its side for imaging, as shown in the lower panel of Figure 2A.
Images from both the PBS and GH-treated mice demonstrated an increase in luminescent signal at 20 h post-treatment compared to pre-treatment (Figure 2B). We hypothesize this is due to the mechanical disruption of the biofilm caused by the irrigation of solution. At 20 h post-treatment, the mice were euthanized, and the wound beds and spleens were collected to enumerate CFU/gram of tissue. While the bacterial loads in the wound beds were similar (Figure 2C), bacteria were only detected in the spleens of GH-treated mice (Figure 2D), suggesting disseminated spread of the dispersed bacteria.
Previously, we have shown that GH treatment can break-up aggregates of bacteria, improving CFU counts21. This may explain the slight increase of bacterial load in the wound beds of the GH-treated mice. Lastly, although the GH-treated wound beds had slightly higher CFU/gram, there was a lower percent bioluminescent change. These results suggest the importance of confirming luminescence with CFU counts. These representative results give an example of data that can be collected by utilizing the protocols described above.
Figure 1. Establishing biofilm-associated wound infections and assessing dispersal agent efficacy in vivo. Dressings were checked for tears or loss of adhesion to the skin of the mouse prior to treating. Compromised dressings were replaced. Mice were placed under anesthesia immediately prior to treatment administration. 200 µL of enzyme solution, or a vehicle control, was injected into the space between the wound bed and dressing. The dressing was gently raised using forceps to ensure the entire wound was saturated in solution. The treatment was left on the wound bed for 30 min. After 30 min, the treatment was aspirated with a syringe, and another 200 µL of treatment was added. This was repeated for a total of 3 treatments. To measure dispersal in real time, mice were imaged using IVIS prior to treatment, immediately post-treatment (0 h), 10 h, and 20 h post-treatment. At 20 h post-treatment, the mice were euthanized, and the wound beds and spleens were collected. The samples were rinsed in PBS and then homogenized in fresh PBS two times at 5 m/s for 60 s. The samples were then serially diluted and spot plated. CFU were enumerated and CFU/gram were calculated. The level of dispersal of bacteria from the wound can either by assessed by IVIS or by determining the number of CFU that spread systemically to the spleen. Please click here to view a larger version of this figure.
Figure 2. Representative data demonstrate how to assess dispersal in vivo. Wounds infected with 104 PAO1(pQF50-lux) for 48 h were treated with either PBS, or 10% GH (biofilm dispersal enzyme solution). PBS treatment was used as a vehicle control. The wound beds were treated with 3 x 30 min treatments. Mice were imaged using IVIS prior to treatment, immediately post-treatment, 10 h post-treatment, and 20 h post-treatment. It should be noted that the side-view images are from different PBS and GH-treated mice than those shown in the overhead-view images (A). The ROIs for each pre-treatment wound bed and each 20 h post-treatment wound bed were recorded. Percent change from pre-treatment was calculated as (ROI pre-treatment-ROI post-treatment/ ROI pre-treatment) x 100 (B). At 20 h post-treatment, mice were euthanized. The wound beds and spleens were collected and weighed to determine CFU/gram (C and D, respectively). N=3. Please click here to view a larger version of this figure.
Figure 3. Serial dilution and spot plating. Homogenized sample was vortexed and 100 µL was transferred to a 1.5 mL Eppendorf tube containing 900 µL of PBS. The sample was vortexed and 100 µL was transferred to another 1.5 mL Eppendorf tube with 900 µL of PBS. This process was repeated 5 times. Starting with the last dilution tube, 10 µL of sample was spot plated onto the agar plate in spot 10-8. This was continued up to dilution 10-3. Typically, the 10-3 dilution results in a lawn of bacteria within the spot. Some organs, which have low bacterial loads, may require spot plating up to the 10-1 dilution. Repeat for each tissue sample. Place agar plate in an incubator at 37 °C for 24-48 h, or grow under conditions that are optimal for the bacterial species of interest. To enumerate CFU, count individual colonies at the lowest dilution possible, and multiply by the appropriate dilution factor to determine CFU/mL or use tissue weight to calculate CFU/g of tissue. Please click here to view a larger version of this figure.
Figure 4. Measuring dispersal from infected tissue ex vivo. Mice were euthanized and the infected wound tissue was aseptically removed. The infected tissue was placed into a 2 mL bead mill homogenizing tube and rinsed briefly with PBS. 1 mL of dispersal agent was added and the sample was incubated for 2 h at 37 °C with shaking at 80 rpm. After incubation, the solution containing the dispersed cells was removed and placed into a separate 1.5 mL tube. The remaining tissue was rinsed with 1 mL of PBS. 1 mL of fresh PBS was added to the tissue and homogenized at 5 m/s for 60 s. The enzyme solution and homogenized tissue were then serially diluted and spot plated onto selective agar. The CFU were enumerated and dispersal was calculated as: CFU from enzyme solution/ (CFU from enzyme solution + CFU from tissue) x 100. Please click here to view a larger version of this figure.
Supplemental Figure 1. Measuring in vivo dispersal. Wounds were infected with 104 PAO1(pQF50-lux) for 48 h and then treated with either PBS, or 10% GH (biofilm dispersal enzyme solution). PBS treatment was used as a vehicle control. The wound beds were treated with 3 x 30 min irrigation treatments. Mice were imaged using IVIS prior to treatment, immediately post-treatment, 10 h post-treatment, and 20 h post-treatment (A). At 20 h post-treatment, mice were euthanized. The wound beds and spleens were collected and weighed to determine CFU/gram (B and C, respectively). N=3. Please click here to download this File.
Here we describe protocols that can be utilized to study the efficacy of biofilm dispersal agents. These protocols can be easily adapted to use with different types of dispersal agents, bacterial species or ex vivo samples, including clinical debridement samples. This protocol also provides a clinically relevant model to collect and study dispersed bacterial cells. The phenotypes of dispersed bacterial cells have been shown to be distinct from those of either planktonic or biofilm cells 5,24,25,26; however, the phenotypes of bacteria dispersed in vivo have yet to be described. If dispersal agents are to be used therapeutically, determining their efficacy, as well as understanding the phenotype that these cells adopt, is important. Additionally, this protocol could be used to study how variations in EPS structure or alteration of signaling pathways affect dispersal. For example, dispersal of P. aeruginosa strains with mutations in different exopolysaccharide components (e.g., Pel, Psl, Alginate) could be compared to that of wild-type strains.
Overall, this protocol can be broken into four main sections: (1) establishing an infection (2) administering treatment or (3) collecting infected tissue for ex vivo treatment and (4) determining dispersal efficacy. The critical part of section 1 is the successful establishment of the infection in the wound bed. If luminescent strains, which require antibiotics to maintain plasmid stability, are being utilized it is imperative to add the appropriate antibiotics when culturing the strains in preparation for infection. It should also be considered that the plasmids may not be maintained during infection in the animal (without antibiotic pressure), thus assessment of bacterial load should not completely rely on IVIS imaging, but be confirmed by CFU. The inoculating dose is also a critical step to initiate infection. For P. aeruginosa and S. aureus, an infecting dose of 104-5 CFU is typically used, but the optimal dose may differ depending upon the bacterial and mouse species used.
The critical part of section 2 is ensuring the wound bed is covered by the dressing throughout the development of infection and treatment. If the dressing is not adhered properly, the treatment can leak from the wound bed and may be ineffective. The dressing is also important for keeping the wound bed protected from potential contaminants, and impairing contractile healing by the mouse, which is essential for mimicking human wound healing. Lastly, the critical part of section 3 is to properly handle the collected samples. To enumerate CFU, the infected tissue should be rinsed with 1 mL of PBS prior to the addition of another 1 mL of PBS for homogenization. More fibrous organs, such as the lungs, may require more thorough homogenization.
The primary limitations of these protocols include the required close monitoring of the dressing and the utilization of IVIS to image dispersal and make assumptions about dispersal efficacy. For long-term studies, the re-growth of fur can be problematic because it can impede dressing adherence to the wound site. Dressings often require replacement, and their removal can lead to accidental debridement of the wound and opportunity for contamination. To visualize dispersal from the wound bed, a bioluminescent bacterial strain must be used. If the bacterial species of interest lacks a bioluminescent reporter, this option is not feasible. Another limitation is the potential of the dispersal treatment to induce bacteremia, which can lead to death of the mouse. However, we have seen that dispersed cells can be effectively killed by antibiotics in vivo7.
The representative results depicted in Figure 2 illustrate a limitation of IVIS for measuring dispersal efficacy. First, a luminescent strain of the bacterial species of interest must be utilized. The strain must produce a luminescent signal that is bright enough to detect through multiple layers of tissue in order to visualize dispersal from the wound bed to other parts of the body. It has been suggested that a minimum of roughly 106 bacteria are required to produce a luminescent signal strong enough to detect by IVIS27. A decrease in luminescent signal has also been described when bacteria enter stationary phase28. This can cause a disconnect between ROIs measured and CFU enumerated29. This limitation can lead to false assumptions when there is not a signal strong enough for detection, but bacteria are still present. As shown in Figure 2B, the PBS and 10% GH treatments resulted in increased luminescence post-treatment. This is a curious and consistent observation, which has also been seen after dispersing cells in vitro (Cláudia Marques, Binghamton University, personal communication). It is possible that the physical disruption of the biofilm from administering the treatment simply spreads out cells that were tightly packed, resulting in increased light signal. Alternatively, dispersal treatment could 'awaken' biofilm cells which were previously metabolically inactive, resulting in increased bioluminescence. Either way, it is crucial to confirm the results of IVIS imaging with CFU data. Lastly, there is a heterogeneous distribution of bacteria within infected wound tissue30,31. Therefore, if the tissue is divided ex vivo to provide more samples, it should not be assumed that the bacterial load in each wound section is identical.
There are modifications and troubleshooting that can be conducted to overcome the limitations of these models. First, the amount of time allotted for exposure of the depilatory cream may need to be adjusted depending upon the mouse species used. For example, 7 min is typically used for Swiss Webster mice, while C57BL/6 mice may need up to 10 min of exposure to successfully remove the fur. This of course also depends on the product used, and exposure times should never exceed the manufacturer's instructions. If a long study (4+ days) is being conducted, depilatory cream may need to be reapplied to remove re-grown fur and ensure a proper seal of the dressing. Next, if a luminescent signal is difficult to detect, the exposure time can be increased during IVIS imaging. The inoculating dose and infection time can also be adjusted. It is important to use an infecting dose that is not too high that it overwhelms the mouse, but is also not too low that the bacteria are immediately cleared by the immune response.
In this protocol, we describe treating 48 h-old wound infections with dispersal agents, as this is a time point we have shown infections caused by P. aeruginosa and/or S. aureus are established and biofilm-associated 12,15,16. However, depending upon the bacterial species, other time points may be more optimal. Ideally, the bacterial load in the wound should plateau once an infection is established. For example, we have seen that with both P. aeruginosa and S. aureus, the bacterial load in the wound reaches approximately 108 CFU/g of tissue by 48 h post-infection and remains at that level until wound closure. Figure 1 illustrates treatment with a solution requiring 3 x 30 min washes. However, if the dispersal agent was in another form, the washes would not be necessary. For example, a cream, gel or engineered dressing could simply be placed on top of the wound bed. Lastly, the wound can be infected in a variety of ways including the inoculation of planktonic bacterial cells, preformed biofilms12, or even infected debridement tissue from another animal or human19,32. We have seen that transplanting a preformed biofilm or debridement sample on to the wound bed results in more successful poly-microbial infections than when multiple species are inoculated in their planktonic form12.
Overall, these protocols describe methods to study biofilm dispersal and the evaluation of therapeutic biofilm dispersal agents. These models allow for the development of complex poly-microbial biofilm-associated infections that mimic clinically relevant human infections. Our hope is that these protocols will be utilized and refined to further the development of anti-biofilm dispersal agents for therapeutic use.
The authors have nothing to disclose.
This work was supported by grants from the National Institutes of Health (R21 AI137462-01A1), the Ted Nash Long Life Foundation, the Jasper L. and Jack Denton Wilson Foundation and the Department of Defense (DoD MIDRP W0318_19_NM_PP).
1.5 mL microcentrifuge tube | Fisher | 14823434 | Use to complete serial dilutions of samples |
25G 58 in needle | Fisher | 14823434 | Attaches to 1 mL syringe |
Ampicillin Sodium Salt | Fisher | BP1760-5 | Make a 50 mg/ mL stock solution and add 100 µL to 10 mL of LB broth for both overnight and subculture |
Amylase | MP Biomedicals | 2100447 | Make a 5% w/v solution, vortex- other dispersal agents can be used |
Buprenorphine SR-LAB 5 mL (1 mg/mL) | ZooPharm | RX216118 | Use as pain mainagement- may use other options |
Cellulase | MP Biomedicals | 2150583 | Add 5% w/v to the 5% w/v amylase solution, vortex, activate at 37 °C for 30 min- other dispersal agents can be used |
Depilatory cream | Walmart | 287746 | Use a small amount to massage into the hair follicles on the back of the animal and allot 10 min to remove hair |
Dressing Forceps, Serrated Tips | Fisher | 12-460-536 | Can use other forms of forceps |
Erlenmeyer flasks baffled 125 mL | Fisher | 101406 | Use to grow overnights and sub-cultures of bacteria |
FastPrep-24 Benchtop Homogenizer | MP Biomedicals | 6VFV9 | Use 5 m/s for 60 s two times to homogenize tissue |
Fatal Plus | Vortech Pharmaceuticals | 0298-9373-68 | Inject 0.2 mL intraperitaneal for each mouse |
Homogenizing tubes (Bead Tube 2 mL 2.4 mm Metal) | Fisher | 15340151 | Used to homogenize samples for plating |
Isoflurane | Diamond Back Drugs | ||
Ketamine hydrochloride/xylazine hydrochloride solution C-IIIN | Sigma Aldrich | K4138 | Use as anasethia- other options can also be utilized to gain a surgical field of anasethia |
LB broth, Miller | Fisher | BP1426-2 | Add 25 g/L and autoclave |
Lidocaine 2% Injectable | Diamond Back Drugs | 2468 | Inject 0.05 mL through the side of the marked wound bed area so it is deposited in the center of the mark. Allot 10 min prior to cutting |
Meropenem | Sigma Aldrich | PHR1772-500MG | Make 5 mg/mL to add to the GH solution to apply topically and a 15 mg/mL solution to inject intraperitaneal 4 h prior and 6 h post-treatment |
Non-sterile cotton gauze sponges | Fisher | 13-761-52 | Use to remove the depilatory cream |
PAO1 pQF50-lux bacterial strain | Ref [13] | N/A | PAO1 pgF50-lux was used as the P. aeruginosa strain of interest in this paper's representative results |
Petri dishes | Fisher | PHR1772-500MG | |
Phosphate Buffer Saline 10x | Fisher | BP3991 | Dilute 10x to 1x prior to use |
Polyurethane dressing | Mckesson | 66024007 | Cut the rounded edge off and cut the remaining square into 4 equal sections |
Pseudomonas isolation agar | VWR | 90004-394 | Add 20 mL/L of glycerol and 45 g/mL to water, autoclave, and pour 20 mL into petri dishes |
Refresh P.M. | Walmart | Use on eyes to reduce dryness during procedure. | |
Sterile Alcohol Prep Pads | Fisher | 22-363-750 | Use to clean the skin immediately prior to wounding to disinfect the area |
Straight Delicate Scissors | Fisher | 89515 | Can also use curved scissors |
Swiss Webster mice | Charles River | 551NCISWWEB | Other mice strains can be used |
Syring Slip Tip 1 mL | Fisher | 14823434 | Used to administer drugs and enzyme treatment |