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Misconceptions, or alternative conceptions, are incorrect understandings that students have incorporated into their prior knowledge. The goal of this study was the identification of misconceptions in microbiology held by undergraduate students upon entry into an introductory, general microbiology course. This work was the first step in developing a microbiology concept inventory based on the American Society for Microbiology's Recommended Curriculum Guidelines for Undergraduate Microbiology. Responses to true/false (T/F) questions accompanied by written explanations by undergraduate students at a diverse set of institutions were used to reveal misconceptions for fundamental microbiology concepts. These data were analyzed to identify the most difficult core concepts, misalignment between explanations and answer choices, and the most common misconceptions for each core concept. From across the core concepts, nineteen misconception themes found in at least 5% of the coded answers for a given question were identified. The top five misconceptions, with coded responses ranging from 19% to 43% of the explanations, are described, along with suggested classroom interventions. Identification of student misconceptions in microbiology provides a foundation upon which to understand students' prior knowledge and to design appropriate tools for improving instruction in microbiology.

If we are to teach effectively, tools are needed to measure student learning. A widely used method for quickly measuring student understanding of core concepts in a discipline is the concept inventory (CI). Using the American Society for Microbiology Curriculum Guidelines (ASMCG) for microbiology, faculty from 11 academic institutions created and validated a new microbiology concept inventory (MCI). The MCI was developed in three phases. In phase one, learning outcomes and fundamental statements from the ASMCG were used to create T/F questions coupled with open responses. In phase two, the 743 responses to MCI 1.0 were examined to find the most common misconceptions, which were used to create distractors for multiple-choice questions. MCI 2.0 was then administered to 1,043 students. The responses of these students were used to create MCI 3.0, a 23-question CI that measures students' understanding of all 27 fundamental statements. MCI 3.0 was found to be reliable, with a Cronbach's alpha score of 0.705 and Ferguson's delta of 0.97. Test item analysis demonstrated good validity and discriminatory power as judged by item difficulty, item discrimination, and point-biserial correlation coefficient. Comparison of pre- and posttest scores showed that microbiology students at 10 institutions showed an increase in understanding of concepts after instruction, except for questions probing metabolism (average normalized learning gain was 0.15). The MCI will enable quantitative analysis of student learning gains in understanding microbiology, help to identify misconceptions, and point toward areas where efforts should be made to develop teaching approaches to overcome them.